The profound insight which comparative anatomy has given us into the

significance and nature of organs, and comparative histology into

those of the cells, has naturally excited a desire to penetrate in the

same way the mystery of the elementary structure of the plasm, the

chief active constituent of the cell. The improved methods of modern

cytology, and the great progress which this science of the cell owes

to the microtome and to microchemistry with its delicate coloring

processes, etc., have prompted many observers of the last three decades

to study the finest structural features of the elementary organism,

and on this foundation build hypotheses as to the elementary structure

of protoplasm. In my opinion, all these theoretical ideas, in so far

as they would explain the finer structure of pure plasm, have a very

serious defect; they relate to microscopic structures which do not

belong to the plasm as such (as a chemical body), but to the cell-body

(or cytosoma), the chief active constituent of which is certainly the

plasm. These microscopic structures are not the efficient causes of

the life-process, but products of it. They are phylogenetic outcomes

of the manifold differentiations which the originally homogeneous and

structureless plasm has undergone in the course of many millions of

years. Hence I regard all these "plasma-structures" (the comb, threads,

granules, etc.), not as original and primary, but as acquired and

secondary. In so far as these structures affect the plasm as such, it

must take the name of metaplasm, or a differentiated plasm, modified

by the life-process itself. The true protoplasm, or viscous and at

first chemically homogeneous substance, cannot, in my opinion, have any

anatomic structure. We shall see, when we come to consider the monera,

that very simple specimens of such organisms without organs still

actually exist.

By far the greater part of the plasm that comes under investigation as

active living matter in organisms is metaplasm, or secondary plasm,

the originally homogeneous substance of which has acquired definite

structures by phyletic differentiations in the course of millions

of years. To this modified plasm we must oppose the original simple

primary plasm, from the modification of which it has arisen. The name

"protoplasm," in the narrower sense, could very properly be retained

for this originally homogeneous form of structureless plasm; but, as

the term has now almost lost definite meaning and is used in many

different senses, it is, perhaps, better to call this pure homogeneous

primary plasm _archiplasm_. It is still found--firstly, in the body of

many (but not all) of the monera, part of the chromacea and bacteria,

and the protamœba and protogenes; and, secondly, in the body of many

very young protists and tissue-cells. In the latter case, however,

there is already a chemical differentiation of the inner caryoplasm and

outer cytoplasm. When we examine these young cells under a high power

of the microscope, with the aid of the modern coloring methods, their

protoplasm seems to be perfectly homogeneous and structureless, or, at

the most, there are merely very fine granules regularly distributed in

it which are believed to be products of metabolism. This is best seen

in many of the rhizopods, especially the amœbæ, thalamophora, and

mycetozoa. There are large amœbæ, which thrust out strongly mobile

feet from their unicellular body, broad, flaplike processes of the

naked cell body which constantly change their form, size, and place.

If they are killed and examined with the aid of the best methods of

coloring, it is quite impossible to detect any structure in them; and

this is also true of the pseudopodia of the mycetozoa and many other

rhizopods. Moreover, the slow flowing movement of the fluid protoplasm

shows clearly that there cannot be any composition out of fine fixed

elements in the body. This is particularly clear in those amœbæ

and mycetozoa in which a hyaline, firm, and non-granulated skin-layer

(hyaloplasm) is more or less separated from a dark, softer, and

granulated marrow-layer (polioplasm); as both of them are viscous and

pass into each other without sharp limits, there cannot be any constant

and fixed structural features in them.

Organic life--in its lowest and simplest form--is nothing but a form

of metabolism, and therefore a purely chemical process. The whole

vital activity of the chromacea, the simplest and oldest organisms

that we know, is confined to that process of metabolism which we

call plasmodomism or carbon-assimilation. The homogeneous and

structureless globules of protoplasm, which represent the whole frame

of these primitive protophyta (chroococcus, aphanocapsa, etc.) in

the simplest conceivable way, expend their whole vital power in the

process of self-maintenance. They maintain their individuality by a

simple metabolism; they grow by the addition of fresh plasm obtained

by it, and they split up into two equal globules of plasm when the

growth passes a certain limit--reproduction by clevage, maintenance

of the species. Thus these chromacea have neither special organs, or

organella, that we can distinguish in their simple plasma-bodies, nor

different functions in their life-process; it is wholly taken up with

the primitive work of their vegetal metabolism. We shall see later

on that this is a purely chemical process, something like catalysis

in inorganic combinations; and for this neither special organs nor

fine elementary structures in the plasm are needed. The "end" of their

existence, self-maintenance, is attained just as simply as in the

catalysis of any inorganic compound, or the formation of a crystal in

its mother-water.

If we compare this very rudimentary life-process of the monera with

that of the highly differentiated protists (diatomes, desmidiacea,

radiolaria, and infusoria), the biological distance between them seems

to be immense; and it is, naturally, far greater when we extend the

comparison to the histona, the highly organized metaphyta and metazoa,

in the bodies of which millions of cells co-operate in the work of the

various tissues and organs.

In the great majority of cells--either the autonomous cells of the

protists or the tissue-cells of the histona--we can detect more or less

definite and constant fine structures in the plasm. We must regard

these always as phyletic, secondary products of the life-process,

and so call the differentiated plasm by the name of metaplasm. The

very different interpretations of the microscopic pictures which this

metaplasm affords have led to a good deal of controversy. In this the

desire to discover in these secondary plasma-structures the first

causes of vital action, or the real elementary organella of the cell,

has played a great part. The most important of the theories that

have been formulated are those of the frothy structure, the skeletal

structure, the fibrous structure, and the granulated structure of the

plasm. All these theories of structure apply to plasm in general, but

particularly to its two chief forms, the caryoplasm of the nucleus and

the cytoplasm of the cell-body.

Among the many different attempts to discover a definite structure in

living matter, the theory of the frothy structure (also called the

honeycomb structure) has lately found the most favor. Otto Bütschli,

of Heidelberg, especially, has endeavored, on the basis of many years

of careful study and experiment, to make it the foundation of his

view of the plasm. It is undeniable that the living matter of many

cells shows a delicate structure which may best be compared with

fine soap-suds; innumerable globules are crowded close together in

a fluid, and flatten each other by their pressure into polyhedrical

shapes. In 1892 Bütschli artificially produced fine oil-suds by

beating up cane sugar or potash in olive oil, and then put a small

drop of the stuff in a drop of water under the microscope. The small

particles of sugar then exercised an attractive action by diffusion on

the particles of water; the latter penetrated into the oily matter,

released the sugar, and formed tiny vesicles with it. As the vesicles

of sugar do not mix with oil, they look like cavities isolated on

all sides, and polyhedrically flattened by mutual pressure. The

striking resemblance of this artificially produced "oil soap-suds" to

the natural and microscopically visible structures of many kinds of

plasm is strengthened from the fact that Bütschli, Georg Quincke, and

others, have also observed similar flowing movements in both; and as

these apparently spontaneous movements can be explained physically and

reduced to adhesion, imbibition, and other mechanical causes, there

seemed a prospect of reducing the "vital" movements of the living

and flowing plasm to purely physical forces. Quite recently Ludwig

Rhumbler, of Göttingen, an authority on the rhizopods, has endeavored

to give in this sense a _Physical analysis of the vital phenomena

in the cell_. To-day the froth theory is much the most popular of

the many attempts to detect a fine plasm-structure as the essential

anatomic foundation of an explanation of the physiological functions.

It must be noted, however, that frequently very different phenomena

are confused under this name, especially the coarser froth-formation

by taking up water in the living matter and the invisible hypothetical

molecular structure. Both these must be distinguished from the finer

plasma-structure which is visible under a powerful microscope; but the

limit between them is difficult to determine.

A second view of the finer structure of the plasm, which had been

greatly esteemed before the acceptance of the froth theory, was

formulated in 1875 by Carl Frommann and Carl Heitzmann, and supported

by Leydig, Schwitz, and others. It puts another interpretation on the

net-like appearance of the microscopic plasma-structure. It assumes

that the plasma consists of a skeleton of fine threads or fibrils

combined in the form of a net, and that these spread and cross in the

body of the cell which is filled with fluid. It is also compared to a

sponge, and is said to have a spongy structure. We can artificially

produce such a skeletal structure by, for instance, causing coagulation

in a thick solution of glue or albumin by adding alcohol or chromic

acid. It is unquestionable that there are these "plasma-skeletons" both

in the nucleus and the body of the cell; but they are generally (if not

always) secondary products of organization in the elementary organism

(or cell-organs), not primitive structures of its plasm. Moreover, an

optical transverse action of a froth-structure or honeycomb, examined

as a flat surface in the microscope, shows the same configuration as

a fine skeleton. We can hardly see any difference between the two. We

cannot accept the skeletal formation as a fundamental structure of the

plasm.

As we notice very fine threads in the plasm of many cells, both in

the caryoplasm of the nucleus and the cytoplasm of the cell body,

the cytologist Flemming, of Kiel (1882), believed it was possible to

discover them in the plasm of all cells, and based on this his filar

theory of plasm. He says that we must distinguish two chemically

different kinds of plasm in living matter--the filar (threadlike) and

the inter-filar matter. The fine threads of the former are of different

lengths, and sometimes run separately, at other times are bound in a

sort of net-work (_mitoma_ and _paramitoma_). In certain conditions of

cell-life, especially in indirect cell-division, these filar formations

play a great part; and also in the functions of highly differentiated

cells, such as the ganglionic cells. But in many cases these plasma

threads may be merely parts of a skeletal or frothy structure

(honeycomb walls in section). In any case, we cannot regard the thread

formation as a general elementary structure of plasm; in my opinion, it

is always a secondary phyletic product of living matter, and never a

primary feature of it.

Totally different from the three preceding theories of the finer

structure of the plasm is the granular theory of Altmann (1890). He

supposes that all living matter is originally made up of tiny round

granules, and that these independently living _bioblasts_ are the real

"elementary organisms," the microscopic ultimate individuals; hence

the cells which are formed by the combination of these granules must

be looked on as individuals of the second order. Between the granules

of the granulated substance (the real active living matter) there

is always an inter-granular substance; the granules are regularly

distributed and arranged in these. The granules themselves, or the

bioblasts, are homogeneous, sometimes globular, and sometimes oval,

or of other shapes. However, the distinction between these substances

is quite arbitrary, and neither chemically nor morphologically well

defined. Under the head of granules Altmann throws together the most

different contents of the cell--fat granules, pigment granules,

secretory granules, and other products of metabolism. Hence his

granular theory is now generally rejected. However, there was a

sound idea at the bottom of it--namely, the idea of explaining the

vital properties and functions of living matter by small separate

constituents which make up the plasm, and move in a viscous medium.

But these real elementary parts are not microscopically visible;

they belong to the molecular world, which lies far below the limit

of microscopic power. In my opinion, Altmann's visible granules,

like Flemming's threads and Frommann's skeleton and Bütschli's

honeycomb, are not primary structures, but secondary products of plasma

differentiation.

As the special properties and activities of any natural body depend on

its chemical constitution, and this is, in the long-run, determined

by the composition of its molecules, it is a matter of the greatest

interest in biology to form as clear and distinct an idea as possible

of the nature and properties of the molecules of plasm. Unfortunately,

it is only possible to do this approximately, and to a slight extent.

As the hypotheses of modern structural chemistry on the molecular

formation of complicated organic compounds are often very unsafe,

this is bound to be the case in the highest degree as regards the

albuminoids and, the most important of all, the living matter or plasm.

We have as yet no knowledge of the fundamental features of its very

variable chemical structure. The one thing that bio-chemists have told

us about it is that the molecule of plasm is very large, and made up of

a great number of atoms (over a thousand); and that these are combined

in smaller or larger groups, and are in a state of very unstable

equilibrium, so that the life process itself causes constant changes in

them.

Since the great problem of heredity was forced by Darwin in 1859

into the foreground of general biology, many different hypotheses

and theories of it have been framed. All these have in the end to

trace it to molecular features in the plasm of the germ-cells;

because it is this germ-plasm of the maternal ovum and the paternal

sperm-cell that conveys the characteristics of the parents to the

child. Hence the great progress that has been made recently in the

study of conception and heredity, by means of a number of remarkable

observations and experiments, has been of service to our ideas on

the molecular structure of the plasm. I have dealt with the chief of

these theories in the ninth chapter of my _History of Creation_, and

must refer the reader thereto. In chronological order we have: (1)

the pangenesis theory of Darwin (1868), (2) the perigenesis theory of

Haeckel (1875), (3) the idioplasm theory of Nägeli (1884), (4) the

germ-plasm theory of Weismann (1885), and (5) the mutation-theory of De

Bries (1889). None of these attempts, and none of the later theories

of heredity, has given us a satisfactory and generally admitted idea

of the plasma-structure. We are not even clear as to whether in the

last resort life is to be traced to the several molecules, or to groups

of molecules, in the plasm. With an eye to this latter difference, we

may distinguish the plastidule and micellar theories as two different

groups of relevant hypotheses.

In my essay on "The Perigenesis of the Plastidules" (1875) I formulated

the hypothesis that in the last instance the plastidules are the

vehicles of heredity--that is to say, plasma-molecules which have the

property of _memory_. In this I found support in the ingenious theory

of the distinguished physiologist, Ewald Hering, who had declared in

1870 that "memory is a general property of organic matter." I do not

see still how heredity can be explained without this assumption! The

very word "reproduction," which is common to both processes, expresses

the common character of psychic memory (as a function of the brain). By

plastidules I understand simple molecules; the homogeneous nature of

the plasm in the monera (both chromacea and bacteria and rhizomonera)

and the primitive simplicity of their life-functions do not dispose

us to think that special groups of molecules are to be distinguished

in these cases. Max Verworn has recently (1903) formulated his

biogen-hypothesis in the same sense, as a "critical-experimental study

of the processes in the living matter." He also takes the active

plasma-molecules, which he calls biogens, as the ultimate individual

factors of the life-process, and is convinced that in the simplest

cases the plasm consists of homogeneous biogen-molecules.

The hypothesis of Nägeli (1884) and Weismann (1885) is totally

different from the hypothesis of the plastidules and biogens as

simple molecules of the plasm. According to this, the ultimate "vital

unities" or individual vehicles of the life-process are not homogeneous

plasma-molecules, but groups of molecules, made up of a number of

different molecules. Nägeli calls them _micella_, and assigns them a

crystalline structure. He supposes that these micella are combined

chainwise into micellar ropes, and that the variety of the many forms

and functions of plasm is due to the different configuration and

arrangement of these. Weismann says: "Life can only arise by a definite

combination of different kinds of molecules, and all living matter

must be made up of these groups of molecules. A single molecule cannot

live, can neither assimilate nor grow nor reproduce." I do not see

the justice of this observation. All the chemical and physiological

properties which Weismann afterwards attributes to his hypothetical

_biophora_ may be ascribed to a single molecule just as well as to

a group of molecules. In the simplest forms of the monera (both the

chromacea and the bacteria) the nature of their rudimentary life

can be explained on the one supposition just as well as the other.

Naturally, this does not exclude a very complicated chemical structure

in the large plastidule or biogen as a single molecule. Verworn's

biogen-hypothesis seems to me quite satisfactory when it represents the

primitive molecule of living matter as really the ultimate factor of

life.

The chief process in the evolutionary history of the plasm is its

separation into the inner nuclear matter (caryoplasm) and the outer

cellular matter (cytoplasm). When both kinds of plasm arose by

differentiation from the originally simple plasm of the monera, there

also took place the morphological separation of the nucleus (caryon)

and cell-body (cytosoma or celleus). As these two chief forms of living

matter are chemically different but nearly related, and as they may

in certain circumstances (for instance, during indirect cell-division

and the partial caryolysis connected therewith) enter into the closest

mutual relations, we must suppose that the original severance of the

two substances took place gradually and during a long period of time.

It was not by a sudden bound or transformation, but by a gradual

and progressive formation of the chemical antithesis of caryoplasm

and cytoplasm, that the real nucleated cell (cytos) arose from the

unnucleated cytode (or primitive cell). Both may correctly be comprised

under the general head of _plastids_ (or formative principles), as

"ultimate individualities."

I regard as the chief cause of this important differentiation of the

plasm the accumulation of hereditary matter--that is to say, of the

internal characteristics of the plastids acquired by ancestors and

transmitted to their descendants--within the plastids while their

outer portion continued to maintain the intercourse with the outer

world. In this way the inner nucleus became the organ of heredity

and reproduction, and the outer cell-body the organ of adaptation

and nutrition. I put forward this hypothesis in 1866 in my _General

Morphology:_ "The two functions of heredity and adaptation seem to

be not yet distributed between differentiated substances in the

unnucleated cytodes, but to inhere in the whole of the homogeneous mass

of the plasm; while in the nucleated cell they are divided between

the two active constituents of the cell, the inner nucleus taking

over the transmission of hereditary characters and the outer plasm

undertaking adaptation, or the accommodation to the features of the

environment." This hypothesis was afterwards (1873) confirmed by the

discoveries of Strasburger, the brothers Hertwig, and others, with

regard to cell-cleavage and fertilization; it is particularly supported

by the phenomena of _caryokinesis_(the movement of the nucleus) in

sexual generation. Hence we can understand how it is that in the monera

(chromacea and bacteria), which propagate by simple cleavage, there is

no sexual generation and no nucleus.

The great significance of the nucleus in the life of the cell, as

central organ of heredity, and also probably as "the soul of the

cell," depends chiefly on the chemical properties of its albuminous

matter, the caryoplasm. This one indispensable nuclear element is

chemically akin to the cytoplasm of the cell-body, but differs from it

in certain respects. The caryoplasm has a greater affinity for many

coloring matters (carmine, hæmatoxylin, etc.) than the cytoplasm; and

the former coagulates more quickly and firmly than the latter through

acids (such as acetic and chromic acid). Hence we need only add a drop

of diluted (two per cent.) acetic acid to cells that seem homogeneous

to make perfectly clear the separation between the inner nucleus and

outer body. As a rule, the firmer nucleus then stands out sharply as

a globular or oval particle of plasm; occasionally it has other forms

(cylindrical, conical, spiral, or branched). The caryoplasm seems

to be originally quite homogeneous and structureless, as we find

in many of the protists and many young cells of histona (especially

young embryos). But in the great majority of cells the caryoplasm is

divided into two or more different substances, the chief of them being

chromatin and achromin.

The most common division of the caryoplasm in the cells of the animal

and plant body, and the one of chief significance for their vital

activity, is that into two chemically different substances, which

are usually called chromatin (or nuclein) and achromin (or linin).

Chromatin has a greater affinity for coloring (_chromos_) matter

(carmine, hæmatoxylin, etc.), and so this "colorable nuclear matter"

is particularly regarded as the vehicle of heredity. The achromin (or

achromatin, or linin) is either not at all or less easily colorable,

and is akin to the cytoplasm; in direct cell-division it enters into

close relations with the latter. Achromin is usually found in the

form of slender threads, and hence called "nuclear thread-matter"

(linin). Chromatin is generally found in roundish or rod-shaped

granules (chromosomata), which exhibit very characteristic changes of

form (loop formation, etc.) in indirect cell-division. The chemical,

physiological, and morphological difference between chromatin and

achromin must not be regarded as an original property of cell nuclei

(as is wrongly stated sometimes), but is the outcome of a very early

phylogenetic differentiation in the originally homogeneous caryoplasm;

and this holds also of two other parts of the nucleus--the nucleolus

and centrosoma.

In a good many cells, but by no means universally, we find two other

constituents of the nucleus, which owe their rise to a further

differentiation of the caryoplasm. The nucleolus is a small globular or

oval particle, which may be found singly or in numbers in the nucleus,

and behaves somewhat differently towards coloring matter than the

closely related chromatin. It has a special affinity for acid aniline

colors, gosin, etc. Its substance has, therefore, been distinguished

as _plastin_ or _paranuclein_. The nucleolus is especially found in

the tissue-cells of the higher animals and plants as an independent

constituent; it is wanting in many of the unicellular protists. The

same may be said of the centrosoma, or "central body" of the cell.

This is an extremely small granule, on the very limit of visibility,

the chemical composition of which is not known very well. We should

have paid no attention to this constituent of the cell (distinguished

in 1876) if it did not play an important, and perhaps leading, part

in indirect cell-division. As the "polar body in the division of

the nucleus," the centrosoma exercises a peculiar attraction on the

granules distributed in the cytoplasm, which arrange themselves

radially about this centre. The centrosomata grow independently and

increase by cleavage, like the chromoplasts (chlorophyll particles,

etc.). When they have split up, each of the daughter-microsomata acts

in turn as a centre of attraction on its half of the cell. However, the

great importance which modern cytologists have ascribed to it on this

account is discounted by two circumstances. In the first place, we have

not succeeded, in spite of all efforts, in discovering a centrosoma in

the cells of the higher plants and many of the protists; and, in the

second place, a number of recent chemical experiments have succeeded in

producing centrosomata artificially (for instance, by the addition of

magnesium chloride) in the cytoplasm. Hence many cytologists regard the

centrosoma as a secondary product of differentiation in the cell-body,

not the nucleus.

Two other parts of the nucleus that we find very often, but by no means

universally, in the cells of the animal and plant body are the nuclear

membrane (caryotheca) and the nuclear sap (caryolymph). A large number

of cells--but not all--have the appearance of vesicles, having a thin

skin enclosing a liquid content, the nuclear sap. The achromin then

usually forms a frame-work of threads, with chromatin granules in its

meshes or knots, within this round vesicle. This very thin nuclear

membrane (often only visible as its contour) or caryotheca may be

regarded as the result of surface-strain (at the planes of contact of

caryoplasm and cytoplasm). The watery and usually clear and transparent

nuclear sap (caryolymph) is formed by imbibition of watery fluid (like

the frothy structure of the plasm in general). The separation of the

nuclear membrane and nuclear sap is not a primary property of the

nucleus, but is due to a secondary differentiation in the originally

homogeneous caryoplasm.

Like the caryoplasm of the nucleus, the cytoplasm of the cell-body is

originally a chemical modification of the simple and once homogeneous

plasm (the archiplasm). This is clearly shown by the comparative

biology of the protists, their unicellular organism presenting a much

greater variety of stages of cell-organization than the subordinate

tissue-cells in the bodies of the multicellular histona. However, in

the great majority of cells the cytoplasm is separated into several,

and frequently very numerous, parts, which have received diverse forms

and functions in the division of labor. We then see very conspicuously

the regularity of cell-organization, which is altogether wanting in

the simple homogeneous plasma granules of the monera. As this great

differentiation of the advanced elementary organism is incorrectly

generalized by some recent cytologists and described as a universal

feature of cells, it is necessary to insist explicitly that it is a

secondary phylogenetic development, and is altogether wanting in the

primitive organisms. The complexity of the physiological division

of labor and the accompanying morphological separation of parts is

extremely great in the cytoplasm. When we wish to arrange them in a

few large groups from a general point of view, we may distinguish the

active plasma-formations from the passive plasma-products; the former

are due to a chemical metamorphosis of the living plasm, the latter

lifeless excretions from it.

Under the head of plasm-formations, or products of differentiation

in the cytoplasm, we comprise all formations that are due to partial

metamorphosis of the living cell-body--not lifeless excretions from

it, but living parts of its substance, undertaking special functions,

and therefore chemically and morphologically differentiated from the

primary cytoplasm. One of the commonest differentiations of this

kind is the separation of the firm hyaline skin-layer (hyaloplasm)

from the softer granular marrow-layer (polioplasm); though the two

often pass into each other without clear limits. In most plant-cells

special granules of plasm, mostly globular or roundish, are developed,

called _trophoplasts_, and these undertake the work of metabolism.

To this class belong the amyloplasts, which produce starch (amylum),

the chloroplasts or chlorophyll-granules which form the green

matter (chlorophyll) in the leaf, and the chromoplasts which form

color-crystals of various sorts. In the cells of the higher animals the

myoplasts form the special contractile tissue of the muscles, and the

neuroplasts the psychic tissue of the nerve-matter. On the other hand,

the distinction between the body-plasm (somoplasma) and the germ-plasm

(germoplasma), which serves as the base of Weismann's untenable theory

of the germ-plasm (_cf._ chapter xvi.), is purely hypothetical and

without direct observation to support it.

The infinite variety of parts of the cell which arise as excretions

of the living active cytoplasm, and so must be regarded as lifeless

plasma-products, may be divided into two chief groups--internal and

external. The former are stored within the living cytoplasm, the

latter thrust out from it.

Internal plasma-products of common occurrence are the microsomata,

very small and opaque particles which are generally regarded as

products of metabolism. They consist sometimes of fat, sometimes of

derivatives of albumin, sometimes of other substances of which we do

not know the chemical composition. The same may be said of the large

and variously-colored pigment-granules, which are very common and

determine the color of tissues. Also very common in the cytoplasm are

large accumulations of fat in the shape of oil-globules, fat-crystals,

etc., besides other crystals of a very different sort, partly organic

crystals (for instance, albuminous crystals in the aleuron-granules

of plants), partly inorganic crystals (for instance, of oxalic-acid

salts in many plant-cells, of calcareous salts in many animal-cells).

The watery cell-sap (cytolymph) plays an important part in many of

the larger cells. It is formed by the accumulation of fluid in the

cytoplasm, and is found in its frothy structure. The large empty spaces

which it forms are called vacuoles, with very regularly disposed

alveoles. When the cell-sap gathers in great abundance within the cell,

we get the large vesicular cells which are found in the tissues of the

higher plants, the cartilages, etc.

As external excretions of the living cytoplasm that have acquired

some importance, especially as protective organs, in the majority of

cells, we have first of all the cell-membranes, the firm capsules or

protective skins which enclose the soft cell-body, like a snail in

its house. In the first period of the cell-theory (1838-1859) such an

integument was ascribed to all cells, and often regarded as their chief

constituent; but it was discovered afterwards that this protective

skin is altogether wanting in many (especially animal) cells, and that

it is not found in many when they are young, but grows subsequently.

We now distinguish between naked cells (gymnocytes) and covered cells

(thecocytes). As examples of naked cells we have the amœbæ, and many

of the infusoria, the spores of algæ, the spermatozoa, and many animal

tissue-cells.

The cell-covering (cytotheca) varies very much in size, shape,

composition, and chemical character, especially in the rhizopods

among the unicellular protists. The flint shells of the radiolaria

and diatomes, the chalky cells of the thalamophora and calcocytea,

the cellulose shells of the desmidiacea and syphonea, show the

extraordinary plasticity of the constructive cytoplasm (_cf._ chapter

viii.). Among the histona the tissue-plants are remarkable for the

infinite variety of shape and differentiation of their cellulose

capsules. The familiar properties of wood, cork, bast, the hard shells

of fruit, etc., are due to the manifold chemical modification and

morphological differentiation which the cellulose membrane undergoes in

the tissues of plants. This is less frequently seen in the tissues of

animals; but, on the other hand, the intercellular and the cuticular

matter play a greater part in these.

The intercellular matter, an important external plasma-product, is

formed by the social cells in the tissues of the histona thrusting

out in common firm protective membranes. These protective structures

are very common among communities of protists, in the form of masses

of jelly, in which a number of cells of the same kind are united;

such are the zooglœa of many of the bacteria and chromacea, the

common jelly-like envelope of the volvocina and many diatomes, and the

globular cell-communities of the polycyttaria (or social radiolaria).

The chief part is played by intercellular matter in the body of the

higher animals, in the form of mesenchyma-tissue; the connecting

tissue, cartilages, and bones owe their peculiar property to the

amount and quality of the intercellular matter that is deposited

between the social cells.

When the socially joined epidermic cells at the surface of the

tissue-body thrust forth in common a protective covering, we get the

cuticles, which are often thick and solid armor-plates. In many of

the metaphyta wax and flinty matter are deposited in the cellulose

cuticles. The strongest formation is found in the invertebrate animals,

where the cuticle often determines the whole shape and articulation,

as in the calcareous shells of mollusks (mussel-shells, snail-shells,

cockle-shells, etc.); and especially the coats of the articulata (the

crab's coat of mail, and the skins of spiders and insects).

VII

UNITIES OF LIFE

Units of life--Simple and complex organisms--Morphological and

physiological individuals--Morphonta and bionta--Stages of

individuality: cell, person, stem--Actual and virtual bionta--Partial

and genealogical bionta--Metaphysical individuals--Cells (elementary

organisms)--Cell membranes--Unnucleated cells--Plastids (cytodes

and cells)--Primitive cells and nucleated cells--Organella (cell

organs)--Cell communities (cœnobia)--Tissues of histona--Systems

of organs--Organic apparatus--Histonal individuals (sprouts and

persons)--Articulation of the histona (metamerism)--Stems of the

histona--Animal states.

The dissection of the body of the higher animal and plant into

its various organs soon prompted comparative anatomists to draw a

distinction between simple and complex organisms. Then, when the

cell-theory developed in the course of the last half-century, the

common anatomic groundwork of all living forms was recognized in

the cell; and the conception of the cell as the elementary organism

led to the further belief that our own frame, like that of all the

higher animals and plants, is a cell-state, composed of millions of

microscopic citizens, the individual cells, which work more or less

independently therein, and co-operate for the common purposes of the

entire community. This fundamental principle of the modern cell-theory

was applied with great success by Rudolph Virchow to the diseased

organism, and led to most important reforms in medicine. The cells are,

in his view, independent "life-unities or individual life-centres,"

and the unified life of the whole man is the combined result of the

work of his component cells. In this way the cells are the real

life-unities of the organism. Their individual independence is at once

seen in the permanently unicellular protists, of which several thousand

species are already known to us.

On the other hand, we find among the lower animals and the higher

plants a composition of homogeneous parts, which represents a higher

stage of life-unity. The tree is an individual, but it is made up of

a number of branches or individual sprouts, each of which consists

in like manner of an axial stem with leaves attached. If we detach

such a branch and plant it in the ground, it takes root and grows

into an independent plant. So the coral-stem is made up of a number

of individual animals or persons, each of which has its own stomach

and mouth with a crown of tentacles. Each several coral-individual is

equivalent to a single living polyp (actinia). Thus the stem (_cormus_)

is a higher unity, both in the animal and the plant world. Even the

herds of gregarious animals, the swarms of bees and ants, and the

communities of human beings, are similar unities; with the difference

that the individual persons or citizens are not physically connected,

but held together by common interests. We can, therefore, distinguish

three stages of organic individuality, one building upon the other--the

cell, the person (or sprout), and the stem or state (cormus). Each

higher unity represents an intimate union of lower individuals.

Morphologically, in relation to their anatomic structure, the latter

are independent; but physiologically, in respect of the life-unity of

the whole, they are subordinated to the former.

This relation is quite clear in the familiar examples I have quoted.

But there are other organisms in which this is not so, and where the

question of the real individuality is very difficult to answer.

Thus, fifty years ago, we came to recognize floating animal-stems in

the remarkable siphonophora, or social medusæ, which had hitherto

been regarded as individual animals, or medusæ with a multiplicity

of organs; further study proved that each of these apparent organs

is really a modified medusa, and the whole united structure a stem.

This example throws a good deal of light on the important question of

association and division of labor. The whole floating siphonophoron

is, physiologically considered (in respect of its vital activity), a

harmoniously organized animal with a number of different organs; but

from the morphological point of view (in respect of form and structure)

each dependent organ is really an independent medusa.

It is clear, from these few illustrations, that the question of

organic individuality is by no means so simple as it seems at first

sight, and that it receives different answers according as we look

at the form and structure (morphologically) or the vital and psychic

activity (physiologically). We must, therefore, distinguish at once

between morphological (_morphonta_) and physiological (_bionta_)

individuals. The tree and the siphonophoron are bionta, or individuals

of the highest order, made up of a number of similar branches or

persons, the social morphonta. But, when we further dissect the latter

anatomically into their various organs, and these again into their

microscopic elements, the cells, each branch or person seems to be a

bion, and their cells to be morphonta. Each multicellular organism is,

however, developed in the beginning from a single cell, the stem-cell

(cytula) or fertilized ovum; this is at once a morphon and a bion, a

simple individual both morphologically and physiologically. The whole

process of its development into a multicellular organism consists in a

repeated cleavage of the stem-cell, the resultant cells being joined

in a higher unity, and assuming different forms in consequence of the

division of work.

The complicated modern state, with its remarkable achievements, may be

regarded as the highest stage of individual perfection which is known

to us in organic nature. But we can only understand the structure of

this extremely complex "organism of the highest order," and its social

forms and functions, when we have a sociological knowledge of the

various classes that compose it, and the laws of their association and

division of labor; and when we have made an anthropological study of

the nature of the persons who have united, under the same laws, for the

formation of a community and are distributed in its various classes.

The familiar arrangement of these classes, and the settling of the rank

in the mass and the governing body, show us how this complex social

organism is built up step by step.

But we have to look in the same way on the cell-state, which is made

up from the separate individualities in human society or in the

kingdom of the tissue-animals, or the branches in the kingdom of the

tissue-plants. Their complex organism, composed of various organs and

tissues, can only be understood when we are acquainted with their

constituent elements, the cells, and the laws according to which these

elementary organisms unite to form cell-communities and tissues, and

are in turn modified in the divers organs in the division of labor. We

must, therefore, first establish the scale of the morphonta, and the

laws of their association and ergonomy, according to which the several

stages or conditions of morphological individuality build on each

other. Three such stages may be at once distinguished: (1) the cell

(or, more correctly, the plastid), (2) the person (animal) or branch

(vegetal), and (3) the stem or cormus. But we shall find that there

are further subordinate stages under each of these three. It is only

in the case of the protists that the morphological unity is bound up

with the physiological. In the case of the histona, the multicellular,

tissue-forming organisms, this is only so at the beginning of

individual existence (at the stage of the stem-cell). As soon as the

multicellular body arises from this cytula by repeated segmentation, it

is raised to the stage of a higher individuality, the cell-state.

Our own human frame is, in its mature condition, like that of all the

higher animals, a very complete cell-state, but a single cell at the

beginning of its existence. We speak of the life-unity of the former

as an actual bion, and that of the latter as a virtual bion; in other

words, the physiological individual or the life-unity has in the first

case reached the highest stage of individual development that pertains

to its species, while in the second case it remains at the lowest

stage of virtual formation, and has only the capacity of rising to

the higher stage. In the higher plants and animals only one cell of

the organism, or the two combined sexual cells (ovum and spermium),

are the potential bion which may develop into an actual one. There

are, however, exceptions. In the fresh-water polyp (hydra) and cognate

cnidaria each piece of the body-wall, in the bath-sponge (euspongia)

and similar sponges each piece of tissue, and in many plants (for

instance, marchantia among the crytogams and bryophyllum among the

phanerogams) each portion of a branch or leaf, has the power to develop

into a mature organism, and is, therefore, a virtual bion.

From these virtual bionta (parts of the body that may grow into whole

organisms) we must distinguish the partial bionta which have not this

property. These are separated parts of the body that live for a time

after being cut off from the whole organism, but then die off. Thus,

for instance, the heart of a tortoise beats for a long time after being

cut out. A flower that has been plucked may, if put in water, keep

fresh and alive for many days. In some highly organized cephalopods one

of the eight arms of the male develops into an independent body, swims

about, and accomplishes the fertilization of the female (_hectocotylus_

among the _argonauta_, _philonexis_, etc.). It was at first thought

to be an independent animal parasite. The same thing happens with

the remarkable foldlike dorsal appendages of a large naked snail

(_thetys_), which get detached and creep about. The body of many of

the lower animals may be cut in pieces and yet may live for weeks. The

life-properties of these partial bionta are important in view of the

general question of the nature of life and its apparent unity in most

of the higher organisms. As a fact, even here the cells and organs lead

their separate individual life, though they are subordinate to and

dependent on the whole.

It has been attempted to answer this question of organic individuality

in the sense of counting all organisms individuals which develop from

a single fertilized ovum. Thus, the Italian botanist Gallesio, in

1816, regarded all plants that arise by asexual generation (budding

or segmentation)--sprouts, branches, slips, bulbs, etc.--as merely

portions of a single individual that came from an egg (the seed).

So also Huxley, in 1855, considered the sum of all the animals that

have been produced by asexual propagation, but from a single sexually

generated animal, to be parts of one individual. In practice, however,

this principle is useless. We should have to say that the millions

of plant-lice which arise parthenogenetically from unfertilized

germ-cells, but are originally descended from one impregnated ovum,

are one single individual; so also all the weeping-willows in Europe,

because they all came from shoots of one single sexually-produced tree.

Many attempts have been made in the course of the nineteenth century

to give a generally satisfactory answer to this difficult question of

the content and connotation of the idea of the organic individual.

None of these has found general favor. I have compared and criticised

them in the third book of my _General Morphology_. I there paid

special attention to the views of Goethe, Alexander Braun, and Nägeli

among the botanists, and Johannes Müller, Leuckart, and Victor Carus

among the zoologists. When we consider the striking divergence of the

views of such distinguished scientists and thinkers on so important a

biological question, we can understand that opinions are still very

divided to-day. Hence we must not be too hard on the metaphysical

philosophers when--in complete ignorance of the real facts--they rear

the most extraordinary theories in their airy speculations on "the

principle of individuation". Compare, for instance, the opinions of the

school-men and those of recent thinkers such as Arthur Schopenhauer and

Edward Hartmann. As a rule, the psychological side of the problem--the

question of the individual soul--is very prominent, without much

attention being paid to its material substratum--the anatomic basis of

the organism. Many metaphysicians, who, in their one-sided anthropism,

make man here also the measure of all things, would assign personal

consciousness as the basis of the idea of individuality. It is obvious

that this is not a practicable test even for the higher animals, to

say nothing of the lower animals and plants. In these we have a far

greater variety of individuality on the one hand, and a far greater

simplicity of construction on the other. I have tried to show, in my

essay on "The Individuality of the Animal Body" (1878), the easiest way

to answer these complicated tectological questions, and to support it

by the science of structure. It suffices to distinguish the three chief

stages I have mentioned, and to explain clearly their physiological

significance on the one hand and morphological on the other. We will

therefore consider the cell first, then the person (or sprout), and,

finally, the stock (or cormus).

Ever since the middle of the nineteenth century the cell theory has

been generally and rightly considered one of the most important

theories in biology. Every anatomical, histological, physiological, and

ontogenetic work must build on the idea of the cell as the elementary

organism. Nevertheless, we are still very far from having a general

and clear agreement as to this universal and fundamental idea. On

the contrary, the ablest biologists still differ considerably as to

the nature of the cell or the elementary individual, its relation to

the whole of the multicellular organism, and so on. This divergence

of views is partly due to the intricacy of the phenomena we find in

the life of the cell, and partly to the many and extensive changes

that have been made in the meaning of the term in the course of its

employment. Let us first cast a glance at the various stages of its

history.

When in the last third of the seventeenth century a number of

scientists, especially Malpighi in Italy and Crew in England, used the

microscope for the first time in the anatomic study of plant structure,

they noticed a certain build of the tissue that closely resembled the

honeycomb. The closely packed wax cells, filled with honey, of the

hive, which show a hexagonal appearance in section, are like the wood

cells that contain the sap in the plant. It was the great merit of

Schleiden, the real founder of the cell theory, to prove that _all_

the different tissues of plants are originally composed of such cells

(1838). Theodor Schwann soon afterwards proved the same for the animal

tissues; in 1839 he extended the theory to the whole organic world.

Both these scientists regarded the cell as essentially a vesicle, the

firm membrane of which enclosed a fluid content, and a solid smaller

body inside this, which R. Brown had recognized as the nucleus in 1833.

They compared the cell, as a microscopic individual, to an organic

crystal, and thought it arose by a sort of crystallization in an

organic medium (cytoblastema); in this the central nucleus would serve

as starting-point like the nucleus of the crystal.

In the first twenty years (1839-59) of the cell theory it was a fixed

principle that there were three essential parts of the cell. Firstly,

there was the strong outer membrane, which was not only regarded as a

protective covering, but also credited with a great deal of importance

as an element in the building of the organism. In the second place,

there was the fluid or semi-fluid content (the sap); and, thirdly,

the firm nucleus enclosed in the sap. In order to give a clearer idea

of the relative thickness and disposition of these parts, the cell

was compared to a cherry or a plum. The soft flesh of this fruit

(corresponding to the cell sap) can, with difficulty, be separated from

the external firm skin or from the hard stone within. A great step in

advance was made in 1860, when Max Schultze showed that the external

membrane was an unessential and secondarily formed part of the cell.

It is, as a fact, altogether wanting in many, especially young, cells

of the animal body. They are naked cells without any membrane. The

distinguished anatomist also proved that the so-called "cell sap"--the

real body of the cell--is not a simple fluid, but a viscous, albuminous

substance, the independent movements of which had long been known

in the rhizopods, and which the first to study it carefully, Felix

Dujardin, had described as _sarcode_ in 1835. Max Schultze further

showed that this "sarcode" was identical with the "cell mucus" of

the plant cells which Hugo Mohl had designated "protoplasm" in 1846,

and that this living matter must be regarded as the real vehicle

of the phenomena of life. As the membrane was now recognized to be

non-essential, of secondary growth, and completely wanting in some

cases, there remained only two essential parts of the cell--the outer

soft cell body, consisting of protoplasm, and the inner firm nucleus,

consisting of a similar substance called nuclein. The original naked

cell was now like a cherry or plum without the skin. This new idea of

the cell, formulated forty years ago, which I endeavored to confirm

in my monograph on the radiolaria (1862), is now generally accepted,

and the cell is defined as a granule or particle of protoplasm (=

cytoplasm) enclosing a firm and definite nucleus (or caryon, consisting

of caryoplasm).

This would be a good occasion to glance at the errors to which

microscopic investigation and the conclusions based on it are liable.

Although Kölliker in 1845, and Remak in 1851, had drawn attention to

the existence of naked cells, and had compared their movements (for

instance, in lymph-cells) to those of the protoplasm in plant-cells,

the majority of the leading microscopists clung for twenty years to

the dogma that every cell must have a membrane; the definite outline

which even a naked cell must show in a different refracting medium was

taken to be the sign of a special and anatomically separable membrane.

It would be just as correct to talk of a protective membrane on a

homogeneous glass ball; its outline is sharply defined. In the long

controversy that "exact" observers sustained as to the presence or

absence of a membrane, this optical error--the false interpretation

of a sharp contour--counted for a good deal. It is much the same

with other conflicts of "exact" observers who give their "certain

observations" as facts, whereas they are really inferences from

imperfect observations on which different interpretations may be put.

Forty years ago (1864) I tried in vain to detect a nucleus in the

naked, living, mobile protoplasm of a few small rhizopod-like protists

(protamœba and protogenes). Other observers, who afterwards studied

similar unnucleated cells (Gruber, Cienkowski, and others), were no

more successful. On the ground of these observations, which were often

repeated afterwards, I formed the class of the _monera_--the simplest

unnucleated organisms--in my _General Morphology_ in 1866, and pointed

out their great importance in solving some of the chief problems of

biology. This importance has been much enhanced of late, since the

chromacea and bacteria have also been recognized as unnucleated cells.

Bütschli has, it is true, raised the objection that their homogeneous

plasma-body behaves, not as cytoplasm, but as caryoplasm (or nuclein),

and so that these simplest plastids correspond, not to the cell-body,

but to the nucleus of other cells. On this view the bacteria and

chromacea are not cells without nuclei, but nuclei without cell-bodies.

This idea agrees with my own in conceiving the plasma-body of the

monera (apart from its molecular structure) as homogeneous and not yet

advanced as far as the characteristic differentiation of inner nucleus

and outer cell-body. Bearing in mind that these essential parts of

the cell (in the view of most cytologists) are chemically related yet

different from each other, we have three possible cases of the original

formation of the nucleated cell from the unnucleated cytode: (i) The

nucleus and cell-body have arisen by differentiation of a homogeneous

plasm (monera); (2) the cell-body is a secondary growth from the

primary nucleus; (3) the nucleus is a secondary development from the

cell-body.

On the first view, which I hold, the plasm, or living matter, of

the earliest organisms on the earth (which can only be conceived as

archigonous monera) was a homogeneous _plasson_ or archiplasm--that

is to say, a plasma-compound that was not yet differentiated into

outer cytoplasm and inner caryoplasm. The rise of this chemical

distinction--and the accompanying morphological division of cell-body

and nucleus--was due to a phyletic differentiation; it was the outcome

of a very early and most important division of labor. The hereditary

matter gathered in the nucleus, the outer cell-matter controlling the

intercourse with the external world. Thus, by this first ergonomy, the

nucleus became the vehicle of heredity and the cell-body the organ of

adaptation. Opposed to this view is the second, the hypothesis which

the founder of the cell-theory, Schleiden, had put forward--that the

nucleus is the original base of the cell, and the cell-body a secondary

development from it. This opinion (which, in the main, corresponds

to that of Bütschli) raises a number of difficulties; as does also

the third hypothesis, that the unnucleated "protoplasm-body" (the

outer cytoplasm-body) is the original formation, and that the nucleus

arose secondarily by condensation and chemical modification of it. At

the bottom, however, the difference between the three hypotheses on

the primary cytogenesis is not as great as it seems at first sight.

However, I am more inclined to adhere to the first; it supposes

that the physiological and chemical differences between nucleus and

cell-body, which afterwards became so important, were not originally

present. The phenomena of caryolysis in indirect cell-division show us

still how close are the relations of the two substances.

If the organic population of our planet has arisen naturally, and not

by a miracle, as Reinke and other vitalists suppose, the earliest

elementary organisms, produced by the chemical process of archigony

(spontaneous generation), could not be real nucleated cells, but

unnucleated cytodes of the type of the chromacea (_cf._ chapter ii.).

The nucleated real cell, as Oscar Hertwig and others define it to-day,

can only have arisen by phylogenetic differentiation of nucleus and

cell-body from the simple cytode of the monera. In that case it is a

matter of simple logic to distinguish the older cytode from the later

cell. The two may then best be comprised (as I proposed in vain in

1866) under the name of "plastids" (formative principles)--that is,

the elementary organism in the broader sense. But if it is preferred

to call the latter _cells_ (in the broader sense), the wrong modern

idea of the cell must be altered, and the nucleus-feature omitted from

it. The cell is then simply the living particle of plasm, and its two

stages of development must be described by other names. The unnucleated

plastid might be called _primitive cell_ (protocytos), and the ordinary

nucleated one the nuclear cell (caryocytos).

A long gradation of cellular organization leads from the simplest

primitive cells (monera) to the highest developed protists. While no

morphological organization whatever is discoverable in the homogeneous

plasma-body of the chromacea and bacteria, we find a composition from

different parts in the highly differentiated body of the advanced

protophyta (diatomes, siphonea) and protozoa (radiolaria, infusoria).

The manifold parts of the unicellular organism, developed by division

of work in the plasm, discharge various functions, and behave

physiologically like the organs of the multicellular histona. But

as the idea of "organ" in the latter is morphologically fixed as a

multicellular part of the body, made up of numerous tissues, we cannot

call these similarly functioning parts "organs of the cell," and had

better describe them as organella (or organoids).

The great majority of the protists are, in the developed condition,

as actual individuals, equivalent morphologically to real nucleated

cells. By means of adaptation to the most varied conditions and

the inheritance of the properties thus acquired such a variety of

unicellular forms has been evolved in the course of millions of

years that we can distinguish thousands of living species, both of

plasmodomous protophyta and plasmophagous protozoa. The number of

known and named species is already as high as this in several distinct

classes, as, for instance, in the diatomes of the primitive plants

and the radiolaria of the primitive animals. These solitary living

unicellulars, or "hermit-cells," may be called _monobia_.

Many other protists have abandoned this original solitary life; they

follow their social instincts and form communities or colonies of cells

(_cœnobia_). These are usually formed by the daughter-cells which

arise from the cleavage of a mother-cell remaining united after the

division, and so on with the succeeding generations which come from

their repeated segmentation. The following are the chief forms of these

cœnobia:

1. GELATINOUS CŒNOBIA.--The social cells secrete a structureless

mass of jelly, and remain associated in the common gelatinous mass,

without actual contact. Sometimes they are regularly, at other times

irregularly, distributed in it. We find cœnobia of this kind even

among the monera, such as the _zooglœa_ of many bacteria and

chromacea. They are common among the protophyta and protozoa.

2. SPHERICAL CŒNOBIA.--The cell-community forms a sort of ball,

the cells lying close together at its surface, touching each other or

even forming a continuous layer; such are _holosphæra_ and _volvox_

among the protophyta, _magosphæra_ and _synura_ among the protozoa.

The latter are particularly interesting because they resemble the

_blastula_, an important embryological stage of the metazoa, of which

the simple, epithelial cell-layer at the surface of the hollow sphere

is called the _blastoderm_ (or germinal membrane).

3. ARBOREAL CŒNOBIA.--The cell-community takes the form of a small

tree or shrub, the fixed cells secreting jelly-like stalks at their

base and these forming branches. At the top of each stalk or branch

is an independent cell; so in the case of the _gomphonema_ and many

other diatomes, the _codonocladium_ among the flagellata, and the

_carchesium_ among the ciliata.

4. CATENAL CŒNOBIA.--The cell-community forms a chain, the links of

which (the individual cells) are joined in a row. We find chainlike

cell-communities of this sort, or "articulated threads," even among the

monera (_oscillaria_ and _nostic_ among the chromacea, _leptothrix_

among the bacteria). Among the diatomes we have the _bacillaria_, among

the thalamophora _nodosaria_, as examples. Many of the lower protophyta

(algaria and algetta) form the direct transition to the true algæ among

the metaphyta, as the threadlike layer of the latter (for instance,

_cladophora_) is only a higher development of the catenal cœnobium,

with polymorphism of the co-ordinated cells. We may also regard these

articulated multicellular threads as the first sketch for the formation

of tissues in the metaphyta.

The stable communities of cells which make up the body of the histona,

or multicellular plants and animals, are called tissues (_tela_ or

_hista_). They differ from the cœnobia of the protists in that

the social cells give up their independence, assume different forms

in the division of labor, and subordinate themselves to the higher

unity of the organ. However, it would be just as difficult to lay

down a sharp limit between the cœnobia and the tissues as between

the protists and the histona which possess them; the latter have been

developed phylogenetically from the former. The original physiological

independence of the cells which have combined to form tissues is more

completely lost in proportion to the closeness of their combination,

the complexity of their division of labor, and the differentiation

and centralization of the tissue-organism. Hence the various kinds of

tissue in the body of the histona behave like the various classes and

professions in a state. The higher the civilization and the more varied

the classes of workers, the more they are dependent on each other, and

the state is centralized.

In the lower tissue-forming plants, the algæ and fungi, the plant-body

has the appearance of a layer of cells, the tissues of which show

little or no division of labor. In these _thallophyta_ there are

none of the conducting or vascular fibres, the formation of which

is of great importance in the higher plants in connection with

their physiological function of circulation of the sap. These more

advanced vascular plants comprehend the two great groups of ferns

(_pteridophyta_) and flowering plants (_anthophyta_, or phanerogams).

Their body is always composed of two chief organs, the axial stem

and the lateral leaves. This is also the case with the mosses

(_bryophyta_), which have no vascular fibres; they lie between the

two chief groups of the non-vascular thallophyta and the vascular

cormophyta. However, this histological and organological division of

the two great groups of tissue-plants must not be pressed; there are

many exceptions and intermediate forms. In general their manifold

tissue-forms may be brought under two chief groups, which we may call

primary and secondary. The primary tissues are the phylogenetically

older and histologically simple "cell-tissues," such as we have in the

thallophyta (algæ, fungi, and mosses); in these there are no conducting

fibres, or, at least, only rudimentary ones. The secondary tissues

are a later development from these; they form conducting and vascular

fibres and other highly differentiated forms of tissue (cambium, wood,

etc.). They make up the bodies of the more complex vascular plants, the

ferns and flowering plants.

In the bodies of the tissue-animals we may similarly distinguish two

chief groups of tissues, the primary and secondary. The former are

phylogenetically and ontogenetically older than the latter. The primary

tissues of the metazoa are the _epitelia_, simple layers of cells or

forms of tissue directly derived from such (glands, etc.). Secondary

tissues, evolved from the former by physiological change of work and

morphological differentiation, are the _apotelia_; of these "derivative

tissues" we may distinguish the three leading groups of connective

tissue, muscular tissue, and nerve tissue. These three great groups of

tissue in the animal world may be subdivided, like the plant groups,

into lower and higher sub-sections. The cœlenteria (gastræads,

sponges, cnidaria) are predominantly built up of epitelia, as are also

the phyletically older group of the cœlomaria; in the vast majority

of the latter, however, the great mass of the body is formed of

apotelia, and they are subject to the most extensive differentiation.

The embryo of all the metazoa consists solely of epitelia (the

germ-layers) at first; apotelia are developed from these afterwards by

differentiation of the tissues.

Comparative anatomy distinguishes in the multicellular body of the

tissue-forming organisms a great number of different parts, which are

regularly adapted to discharge definite vital functions, and have been

most intricately developed in virtue of the division of labor. They are

called "organs" in the stricter sense in opposition to the organella

(or organoids) of the protists; the latter have, it is true, a similar

physiological purport, but are not (being parts of a cell) equal to the

former morphologically. The remarkable efficiency that we find in the

structure of the various organs in view of the functions they have to

discharge, and the regularity of their construction in the unity of

the histon--in other words, their adaptive organization--is explained

mechanically by the theory of selection, while the teleological

hypotheses of dualistic biology (for instance, the "intelligent

dominants" of Reinke) completely fail to account for their origin. The

gradual advance of the organs and their physiological division of labor

have many analogies in the two kingdoms of the histona. While at the

lowest stages the simple organ represents only a separate individual

piece of primitive tissue, we find special systems of organs and

organic apparatus in the higher stages.

The idea of a particular system of organs is determined by the unity

of one tissue which forms the characteristic element in the totality

of the organs that belong to it. Of such systems in the kingdom of the

metaphyta we have: the skin-system (with the tissue of the epidermis),

the vascular system (with its conducting and vascular fibres), and the

complementary tissue system (with the basic tissue). In the kingdom of

the metazoa we may similarly distinguish: the skin-system (integument

of the epidermis), the vascular system (with the mesenchyma-tissue

of the blood and blood-vessels), the muscular system (with the

muscle-tissue), and the nervous system (with the neurona of the

nerve-tissue).

In contrast with the histological idea of a system of organs, we have

the physiological conception of an apparatus of organs. This is not

determined by the unity of the constituent tissue, but by the unity of

the lifework that is accomplished by the particular group of organs in

the histona. Such an apparatus of organs is, for instance, the flowers

and the fruit developing therefrom in the phanerogams, or the eye or

the gut of an animal. In these apparatus the most diverse organs and

systems of organs may be associated for the fulfilment of a definite

physiological task.

In the higher animals and plants we usually regard as the "real

individual" (in the wider sense of the word) the tissue-forming

organism made up of various organs; and we may here briefly and

instructively call this the histonal individual (or, more briefly,

the "histonal"). Botanists call this individual phenomenon among

the metaphyta a sprout (_blastus_). Zoologists give the title of

"person" (_prosopon_) to the corresponding unity among the animals.

The two forms agree very much in their general features, and may be

called "individuals of the second order," if we take the cells to be

the first and the stock the third stage in the hierarchy of organic

individuality. In comprising them here under the general head of

histonals, or histonal individuals, I mean by this to designate the

definite physiological unity of the multicellular and tissue-forming

organism, as contrasted with the unicellular protist on the one hand,

and the higher stem, made up of several histonals, on the other.

The plant-histonal, which Alexander Braun especially clearly marked

out and described as the sprout, is found in two principal forms in

the kingdom of the metaphyta--the lower form of the layer-sprout

(_thallus_) and the higher form of the stalk-sprout (_culmus_).

The thallus predominates in the lower and older sub-kingdom of the

layer-plants (_thallophyta_), in the classes of the algæ and fungi;

the culmus in the higher and younger sub-kingdom of the stalk-plants

(_cormophyta_), in the classes of the mosses, ferns, and flowering

plants. The culmus presents in general the characteristic form of

an axial central organ, the stalk, with lateral organs, the leaves,

attached to this at the sides, the former having an unlimited vertical

growth and the latter an unlimited basal growth. The thallus does not

yet show this important morphological division. There are, however,

exceptions in both groups of the metaphyta. The large and highly

developed _fucoidea_ among the algæ exhibit similar differentiations

of organs to those we distinguish as stalk and leaves in the higher

cormophyta. On the other hand, they are wanting in the lower

liverworts, which form a thallus like many of the algæ; thus, for

instance, the liverwort _riccia fluitans_ is just like the brown

alga _dictyota dichotoma_. Other primitive liverworts (such as the

_anthoceros_) have also a very simple thallus; but most of them have

a separation of the thallus into an axial organ (stalk) and several

lateral organs (leaves). In the distribution of labor among the

leaves there then emerge the differences between the lower leaves,

foliage leaves, higher leaves, and flower leaves. A simple poppy-plant

(_papaver_) or a single-flowered _gentiana ciliata_, which has only one

bloom at the top of its branchless stalk, is a good example of a highly

developed culmus.

To the plant-sprout corresponds in the animal world the _person_. All

the tissue-animals pass in the course of their embryonic development

through the important stage of the _gastrula_, or "cup-shaped embryo."

The whole body of the tissue-animal at this stage forms at first a

simple gut-sac or gastric sac (the primitive gut), the cavity of

which opens outward by a primitive mouth. The thin wall of the sac is

formed by two superimposed layers of cells, the two primary germinal

layers. This gastrula is the simplest form of the "person," and the two

germinal layers are its sole organs.

The diverse animal forms which develop along different lines

from this common embryonic form of the gastrula may be grouped

into two sub-kingdoms, the lower (_cœlenteria_) and the upper

(_cœlomaria_) animals. The former correspond in the simplicity of

their structure in many respects to the thallophyta, and the latter

to the cormophyta. Of the four stems of the cœlenteria (which

have only a ventral opening and no gut-cavity) the gastræads remain

at the gastrula stage, and the sponges are formed by multiplication

of the same stems of gastræads. On the other hand, the cnidaria

develop into higher radial (star-shaped) persons, and the platodes

into lower bilateral persons. From the latter are derived the worms

(_vermalia_), the common stem-groups of the five higher animal stems,

the unarticulated mollusks, echinoderms, and tunicates, and the

limb-forming articulates and vertebrates.

A large part of the physiological advantages and morphological

perfection which the higher histona have, as contrasted with the lower,

may be traced to the circumstance that the tissue-forming organism

articulates--that is to say, divides on its long axis into several

sections. With this multiplication of groups of organs there goes, as a

rule, a more or less extensive division of work among them, a leading

factor of higher development. In this point also we see the biogenetic

parallelism between the two great groups of the tissue-plants and

tissue-animals.

In the kingdom of the tissue-plants the articulated cormophyta rise

high above the unarticulated thallophyta. While the articulation of

the stem of the former proceeds and leaves are developed at the knots

(_nodi_) between each two sections of the stalk, far greater play is

offered to polymorphic differentiation than in the thallophyta, which

are generally without this metamerism. The formation of the bloom in

the flowering plants or phanerogams consists in a sexual division of

labor among the thickly gathered leaves in a short section of a stem.

To the two groups of unarticulated and articulated sprouts in the

kingdom of the tissue-plants correspond, in many respects, the two

sections of the tissue-animals, the unarticulated and the articulated.

The two stems of the articulates and vertebrates rise above all the

other metazoa by the perfection of their organism and the variety

of their functions. In the articulates the metamerism is chiefly

external--an articulation of the body wall. In the vertebrates it

mainly affects the internal organs, the skeleton, and the muscular

system. The vertebration (articulation) of the vertebrates is not

outwardly visible like that of the articulates. In both stems the

articulation is similar in the lower and upper forms, as we find in

the annelids and myriapods, the acrania and cyclostoma. On the other

hand, the higher the organization the greater is the unlikeness of

the members or articulated parts, as in the arachnida and insects,

the amphibia and amniotes. The same antithesis is found in the lower

and higher crustacea. This metamerism of the higher metazoa is of a

motor character, having been acquired through the manner of movement

of the lengthened body; but we find in some groups of the lower, and

usually unarticulated, metazoa a propagative metamerism, determined

by budding at the end; such is the strobilation of the chain-worms

and the scyphostoma polyps. The individual metamera (parts) that are

released from the end of the chain in these cases immediately show

their individuality. This is also the case with many of the annelids,

in which every member that is separated has the power to reproduce the

whole chain of metamera.

The third and highest stage of individuality to which the multicellular

organism attains is the stock or colony (_cormus_). It is usually

formed by a permanent association of histonals that are produced

by cleavage (imperfect segmentation or budding) from one histonal

individual. The great majority of the metaphyta form complex plants in

this sense. But among the metazoa we find this form of individuality

only in the lower (and generally stationary) stages of development.

Here also there is a striking parallelism of development between the

two chief groups of the histona. At the lower stages of stock-formation

there is equality of the social histonals. But in the higher grades

they become unequally developed in the division of labor; and the

greater the differences between them become, the greater is the

centralization of the whole stock (as in the case of the siphonophora).

We may therefore distinguish two principal forms of stocks--the

homonomous and heteronomous, the one without, and the other with,

division of labor among the histonals.

The history of civilization teaches us that its gradual evolution is

bound up with three different processes: (1) Association of individuals

in a community; (2) division of labor (ergonomy) among the social

elements, and a consequent differentiation of structure (polymorphism);

(3) centralization or integration of the unified whole, or rigid

organization of the community. The same fundamental laws of sociology

hold good for association throughout the entire organic world; and also

for the gradual evolution of the several organs out of the tissues

and cell-communities. The formation of human societies is directly

connected with the gregariousness of the nearest related mammals. The

herds of apes and ungulates, the packs of wolves, the flocks of birds,

often controlled by a single leader, exhibit various stages of social

formation; as also the swarms of the higher articulates (insects,

crustacea), especially communities of ants and termites, swarms

of bees, etc. These organized communities of free individuals are

distinguished from the stationary colonies of the lower animals chiefly

by the circumstance that the social elements are not bodily connected,

but held together by the ideal link of common interest.

VIII

FORMS OF LIFE

Morphology--Laws of symmetry--Fundamental forms of animals

and plants--Fundamental forms of protists and histona--Four

chief classes of fundamental forms: (1) Centrostigma: vesicles

(smooth vesicle and tabular vesicle); (2) Centraxonia: typical

forms with central axis--Uniaxial (monaxonia, equipolar and

unequipolar)--Transverse-axial (stauraxonia, double-pyramidal

and pyramidal); (3) Centroplana: fundamental forms with

central plane--Bilateral symmetry--Bilateral-radial and

bilateral-symmetrical fundamental forms--Asymmetrical fundamental

forms; (4) Anaxonia: irregular fundamental forms--- Causes of

form-construction--Fundamental forms of monera, protists, and

histona--Fundamental form and mode of life--Beauty of natural

forms--Æsthetics of organic forms--Art forms in nature.

The infinite variety of forms which we observe in the realm of organic

life not only delight our senses with their beauty and diversity, but

also excite our curiosity, in suggesting the problem of their origin

and connection. While the æsthetic study of the forms of life provides

inexhaustible material for the plastic arts, the scientific study of

their relations, their structures, their origin and evolution, forms

a special branch of biology, the science of forms or morphology. I

expounded the principles of this science in my _General Morphology_

thirty-eight years ago. They are so remote from the ordinary curriculum

of education, and are so difficult to explain without the aid of

numerous illustrations, that I cannot think of going fully into

them here. In the present chapter I will only briefly describe those

features of living things which relate to the difficult question of

their ideal fundamental forms, the laws of their symmetry, and their

relation to crystal-formation. I have treated these intricate questions

somewhat fully in the last (eleventh) part of _Art-forms in Nature_.

The hundred plates contained in this work may serve as illustrations of

morphological relations. In the following pages the respective plates

are indicated by the letters A-f, with the number of each.

The unity of the organic structure, which expresses itself everywhere

in the fundamental features of living things and in the chemical

composition and constructive power of their plasm, is also seen in

the laws of symmetry in their typical forms. The infinite variety of

the species may, both in the animal and plant worlds, be reduced to

a few principal groups or classes of fundamental forms, and these

show no difference in the two kingdoms (_cf._ plate 6). The lily has

the same regular typical form as the hexaradial coral or anemone

(A-f, 9, 49), and the bilateral-radial form is the same in the

violet and the sea-urchin (clypeaster, A-f, 30). The dorsiventral or

bilateral-symmetrical form of most green leaves is repeated in the

frame of most of the higher animals (the cœlomaria); the distinction

of right and left determines in each the characteristic antithesis of

back and belly.

The distinction between protists and histons is much more important

than the familiar division of organisms into plants and animals, in

respect of their fundamental forms and their configuration. For the

protists, the unicellular organisms (without tissue) exhibit a much

greater freedom and variety in the development of their fundamental

forms than the histons, the multicellular tissue-forming organisms. In

the protists (both protophyta and protozoa) the constructive force of

the elementary organism, the individual cell, determines the symmetry

of the typical form and the special form of its supplementation; but in

the histons (both metaphyta and metazoa) it is the plasticity of the

tissue, made up of a number of socially combined cells, that determines

this. On the ground of this tectological distinction we may divide

the whole organic world into four kingdoms (or sub-kingdoms), as the

morphological system in the seventh table shows.

In respect of the general science of fundamental forms (promorphology),

the most interesting and varied group of living things is the class

of the radiolaria. All the various fundamental forms that can be

distinguished and defined mathematically are found realized in the

graceful flinty skeletons of these unicellular sea-dwelling protozoa.

I have distinguished more than four thousand forms of them, and

illustrated by one hundred and forty plates, in my monograph on the

_Challenger_ radiolaria [translated].

Only a very few organic forms seem to be quite irregular, without any

trace of symmetry, or constantly changing their formless shape, as we

find, for instance, in the amœbæ and the similar amœboid cells

of the plasmodia. The great majority of organic bodies show a certain

regularity both in their outer configuration and the construction of

their various parts, which we may call "symmetry" in the wider sense

of the word. The regularity of this symmetrical construction often

expresses itself at first sight in the arrangement side by side of

similar parts in a certain number and of a certain size, and in the

possibility of distinguishing certain ideal axes and planes cutting

each other at measurable angles. In this respect many organic forms

are like inorganic crystals. The important branch of mineralogy that

describes these crystalline forms, and gives them mathematical

formulæ, is called crystallography. There is a parallel branch of the

science of biological forms, promorphology, which has been greatly

neglected. These two branches of investigation have the common aim of

detecting an ideal law of symmetry in the bodies they deal with and

expressing this in a definite mathematical formula.

The number of ideal fundamental forms, to which we may reduce the

symmetries of the innumerable living organisms, is comparatively

small. Formerly it was thought sufficient to distinguish two or three

chief groups: (1) radial (or actinomorphic) types, (2) bilateral (or

zygomorphic) types, and (3) irregular (or amorphic) types. But when

we study the distinctive marks and differences of these types more

closely, and take due account of the relations of the ideal axes and

their poles, we are led to distinguish the nine groups or types which

are found in the sixth table. In this promorphological system the

determining factor is the disposition of the parts to the natural

middle of the body. On this basis we make a first distinction into four

classes or types: (1) the centrostigma have a _point_ as the natural

middle of the body; (2) the centraxonia a straight line (axis); (3) the

centroplana a plane (median plane); and (4) the centraporia (acentra or

anaxonia), the wholly irregular forms, have no distinguishable middle

or symmetry.

I. CENTROSTIGMATIC TYPES.--The natural middle of the body is a

mathematical point. Properly speaking, only one form is of this

type, and that is the most regular of all, the sphere or ball.

We may, however, distinguish two sub-classes, the smooth sphere

and the flattened sphere. The smooth sphere (_holospœra_) is a

mathematically pure sphere, in which all points at the surface are

equally distant from the centre, and all axes drawn through the

centre are of equal length. We find this realized in its purity

in the ovum of many animals (for instance, that of man and the

mammals) and the pollen cells of many plants; also cells that

develop freely floating in a liquid, the simplest forms of the

radiolaria (_actissa_), the spherical cœnobia of the volvocina

and catallacta, and the corresponding pure embryonic form of the

_blastula_. The smooth sphere is particularly important, because it

is the only absolutely regular type, the sole form with a perfectly

stable equilibrium, and at the same time the sole organic form which

is susceptible of direct physical explanation. Inorganic fluids

(drops of quicksilver, water, etc.) similarly assume the purely

spherical form, as drops of oil do, for instance, when put in a

watery fluid of the same specific weight (as a mixture of alcohol and

water).

The flattened sphere, or facetted sphere (_platnosphæra_), is known

as an endospherical polyhedron; that is to say, a many-surfaced body,

all the corners of which fall in the surface of a sphere. The axes

or the diameters, which are drawn through the angles and the centre,

are all unequal, and larger than all other axes (drawn through the

facets). These facetted spheres are frequently found in the globular

silicious skeletons of many of the radiolaria; the globular central

capsule of many spheroidea is enclosed in a concentric gelatine

envelope, on the round surface of which we find a net-work of fine

silicious threads. The meshes of this net are sometimes regular

(generally triangular or hexagonal), sometimes irregular; frequently

starlike silicious needles rise from the knots of the net-work (A-f,

1, 51, 91). The pollen bodies in the flower-dust of many flowering

plants also often assume the form of facetted spheres.

II. CENTRAXONIA TYPES.--The natural middle of the body is a straight

line, the principal axis. This large group of fundamental forms

consists of two classes, according as each axis is the sole fixed

ideal axis of the body, or other fixed transverse axes may also

be distinguished, cutting the first at right angles. We call the

former uniaxial (_monaxonia_), and the latter transverse-axial

(_stauraxonia_). The horizontal section (vertically to the chief

axis) is round in the uniaxials and polygonal in the transverse-axial.

In the monaxonia the form is determined by a single fixed axis,

the principle axis; the two poles may be either equal (_isopola_)

or unequal (_allopola_). To the isopola belong the familiar simple

forms which are distinguished in geometry as spheroids, biconvex,

ellipsoids, double cones, cylinders, etc. A horizontal section,

passing through the middle of the vertical chief axis, divides the

body into two corresponding halves. On the other hand, many of the

parts are unequal in size and shape in the _allopola_. The upper pole

or vertex differs from the basal pole or ground surface; as we find

in the oval form, the planoconvex lens, the hemisphere, the cone,

etc. Both sub-classes of the monaxonia, the allopola (conoidal) and

the isopola (spheroidal), are found realized frequently in organic

forms, both in the tissue-cells of the histona and the independently

living protists (A-f, 4, 84).

In the stauraxonia the vertical imaginary principal axis is cut by

two or more horizontal cross-axes or radial-axes. This is the case in

the forms which were formerly generally classed as regular or radial.

Here also, as with the monaxonia, we may distinguish two sub-classes,

isopola and allopola, according as the poles of the principal axis

are equal or unequal.

Of the _stauraxonia isopola_ we have, for instance, the double

pyramids, one of the simplest forms of the octahedron. This form is

exhibited very typically by most of the acantharia, the radiolaria

in which twenty radial needles (consisting of silicated chalk) shoot

out from the centre of the vertical chief axis. These twenty rays

are (if we imagine the figure of the earth with its vertical axis)

distributed in five horizontal zones, with four needles each, in

this wise: two pairs cross at right angles in the equatorial zone,

but on each side (in north and south hemispheres) the points of four

needles fall in the tropical zone, and the points of four polar

needles in the polar circles; twelve needles (the four equatorial

and eight polar) lie in two meridian planes that are vertical to

each other; and the eight tropical needles lie in two other meridian

planes which cross the former at an angle of forty-five degrees.

In most of the acantharia (the radial acanthometra and the mailed

acanthophracta)--there are few exceptions--this remarkable structural

law of twenty radial needles is faithfully maintained by heredity.

Its origin is explained by adaptation to a regular attitude which

the sea-dwelling unicellular body assumes in a certain stage of

equilibrium (A-f, 21, 41). If the points of the real needles are

connected by imaginary lines, we get a polyhedrical body, which may

be reduced to the form of a regular double pyramid. This typical form

of the equipolar stauraxonia is also found in other protists with a

plastic skeleton, as in many diatomes and desmidiacea (A-f, 24). It

is more rarely found embodied in the tissue-cells of the histona.

Unequipolar stauraxonia are the pyramids, a fundamental form that

plays an important part in the configuration of organic bodies. They

were formerly described as regular or fundamental forms. Such are the

regular blooms of flowering plants, the regular echinoderms, medusæ,

corals, etc. We may distinguish several groups of them according to

the number of the horizontal transverse axes that cut the vertical

main axis in the middle.

Two totally different divisions of the pyramidal types are the

regular and the amphithecta pyramids. In the regular pyramids the

transverse axes are equal, and the ground-surface (or base) is a

regular polygon, as in the three-rayed blooms of the iris and crocus,

the four-rayed medusæ (A-f, 16, 28, 47, 48, etc.), the five-rayed

"regular echinoderms," most of the star-fish, sea-urchins, etc. (A-f,

10, 40, 60), and the six-rayed "regular corals" (A-f, 9, 69).

The amphithecta (or two-edged) pyramids, a special group of

pyramidal types, are characterized by having as their basis a

rhombus instead of a regular polygon. We may, therefore, draw two

imaginary transverse axes, vertical to each other, through the

ground-surface, both equipolar, but of unequal length. One of the

two may be called the sagittal axis (with dorsal and ventral pole),

and the other the transverse axis (with right and left pole); but

the distinction is arbitrary, as the two are equipolar. In this lies

the chief difference from the centroplane and dorsiventral forms, in

which only the lateral axis is equipolar, the sagittal axis being

unequipolar. We find the bisected pyramid in a very perfect form in

the class of the ctenophora (or comb-medusæ, A-f, 27), where it is

quite general. The striking typical form of these pelagic cnidaria

is sometimes called biradial, sometimes four-rayed and bilateral,

and sometimes eight-rayed-symmetrical. Closer study shows it to be a

rhombus-pyramid. The originally four-rayed type, which it inherited

from craspedote medusæ, has become bilateral by the development of

different organs to the right and left from those before and behind.

Similar rhombo-pyramidal forms to those of the ctenophora are also

found in some of the medusæ and siphonophora, many of the corals and

other cnidaria, and many flowers. The name "two-edged" which is given

to this special type is taken from the ancient two-edged sword. Its

chief axis is unequipolar, the handle being at the basic pole and

the point at the verticle pole; but the two edges left and right are

equal (poles of the lateral axis), and also the two broad surfaces

(dorsal and ventral, joined by the sagittal axis).

III. CENTROPLANE TYPES.--The natural middle of the body is a plane,

the median or chief plane (_planum medianum_ or _sagittale_); it

divides the bilateral body into two symmetrical halves, the right

and the left. With this is associated the characteristic antithesis

of back (_dorsum_) and belly (_venter_); hence, in botany this

type (found, for instance, in most green leaves) is called the

dorsiventral, and in zoology the bilateral in the narrower sense. One

characteristic of this important and wide-spread type is the relation

of three different axes, vertical to each other; of these three

straight axes (enthyni) two are unequipolar and the third equipolar.

Hence, the centroplanes may also be called tri-axial (_triaxonia_).

In most of the higher animals (as in our own frame) the longest of

the three axes is the principal one (_axon principalis_); its fore

pole is the oral or mouth pole, and its hinder pole is the aboral

or caudal (tail) pole. The shortest of the three enthyni is, in our

body, the sagittal (arrow) or dorsiventral axis; its upper pole is

at the back and its lower pole at the belly. The third axis--the

transverse or lateral axis--is equipolar, one pole being called the

right and the other the left. The various parts which make up the two

halves of the body have relatively the same disposition in each half;

but absolutely speaking (namely, in relation to the middle plane)

they are oppositely arranged.

Further, the centroplane or bilateral forms are also characterized

by three vertical axes which may be drawn through each of the normal

axes. The first of these normal planes is the median plane; it is

defined by the chief axis and the sagittal axis, and divides the body

into two symmetrical halves, the right and left. The second normal

plane is the frontal plane; this passes through the chief axis and

the transverse axis (which is parallel to the frontal surface in

our body), and divides the dorsal half from the ventral half. The

third normal plane is the cingular (waist) plane: this is defined by

the sagittal and transverse axes. It divides the head half (or the

vertical part) from the tail half (or the basal part).

The name "bilateral symmetry," which is especially applied to the

centroplane and dorsiventral types, is ambiguous, as I pointed out

in 1866 in an exhaustive analysis and criticism of these fundamental

forms in the fourth book of the _General Morphology_. It is used in

five different senses. For our present general purpose it suffices to

distinguish two orders of centroplane types, the bilateral-radial and

the bilateral-symmetrical; in the former the radial (pyramidal) form

is combined with the bilateral, but not in the latter.

The bilateral-radial type comprises those forms in which the radial

structure is combined in a very characteristic fashion with the

bilateral. We have striking examples in the three-rayed flowers

of the orchids (A-f, 74), the five-rayed blooms of the labiate

and papilionaceous flowers, etc., in the plant world; and in the

five-rayed "irregular" echinoderms, the bilateral sea-urchins

(spatangida, clypeastrida, A-f, 30) in the animal world. In these

cases the bilateral symmetry is recognizable at the first glance, as

is also the radial structure, or the composition from three to five

or more raylike parts (paramera), which are arranged bilaterally

round a common central plane.

The bilateral-symmetrical type is general among the higher animals

which move about freely. The body consists of two antithetic parts

(_antimera_), and has no trace of radial structure. In the free

moving, creeping, or swimming animals (vertebrates, articulates,

mollusks, annelids, etc.) the ventral side is underneath, against the

ground, and the dorsal side upward. This form is clearly the most

useful and practical of all conceivable types for the movement of

the body in a definite direction and position. The burden is equally

distributed between the two sides (right and left); the head (with

the sense organs, the brain, and the mouth) faces frontward and the

tail behind. For thousands of years all artificial vehicles (carts on

land and ships in water) have been built on this type. Selection has

recognized it to be the best and preserved it, while it has discarded

the rest. There are, however, other causes that have produced the

predominance of this type in green leaves--the relation to the

supporting stalk, to the sunlight that falls from above, etc.

Special notice must be taken of those bilateral forms which were

originally symmetrical (by heredity), but have subsequently become

asymmetrical (or of unequal halves), by adaptation to special

conditions of life. The most familiar example among the vertebrates

are the flat-fishes (_pleuronectides_), soles, flounders, turbots,

etc. These high and narrow and flattened boney-fishes have a perfect

bilateral symmetry when young, like ordinary fishes. Afterwards they

form the habit of laying on one side (right or left) at the bottom of

the sea; and in consequence the upper side, exposed to the light, is

dark colored, and often marked with a design (sometimes very like the

stony floor of the ocean--a protective coloring), while the side the

flat-fish lies on remains without color. But, what is more curious,

the eye from the under side travels to the upper side, and the two

eyes lie together on one side (the right or left); while the bones of

the skull and the softer parts of each side of the head grow quite

crooked. Naturally, this ontogenetic process, in which a striking lack

of symmetry succeeds to the early complete symmetry of each individual,

can only be explained by our biogenetic law; it is a rapid and brief

recapitulation (determined by heredity) of the long and slow phyletic

process which the flat-fish has undergone for thousands of years in its

ancestral history to bring about its gradual modification. At the same

time, this interesting metamorphosis of the _pleuronectides_ gives us

an excellent instance of the inheritance of acquired characteristics,

as a consequence of constant œcological habit. It is quite

impossible to explain it on Weismann's theory of the germ-plasm.

We have another striking example among the invertebrates in the snails

(_gasteropoda_). The great majority of these mollusks are characterized

by the spiral shape of their shells. This variously shaped, and often

prettily colored and marked, snail's house is in essence a spirally

coiled tube, closed at the upper end and open at the lower (or mouth):

the mollusk can at any moment withdraw into its tube. The comparative

anatomy and ontogeny of the snails teach us that this spiral shell

came originally from a simple discoid or cylindrical dorsal covering

of the once bilateral-symmetrical mollusk, by the two sides of the

body having an unequal growth. The cause of it was a purely mechanical

factor--the sinking of the growing visceral sac, covered with the

shell, to one side; one part of the viscera contained in it (the

heart, kidneys, liver, etc.) grew more strongly on one side than the

other in consequence of this; and this was accompanied by considerable

displacement and modification of the neighboring parts, especially the

gills. In most snails one of the gills and kidneys and the ventricle

of the heart corresponding to these have disappeared altogether,

only those of the opposite side remaining; and the latter have moved

from the right side to the left, or vice versa. The conspicuous lack

of symmetry between the two halves of the body which resulted from

this finds expression in the spiral form of the snail's shell. This

remarkable ontogenetic metamorphosis also can be fully explained by a

corresponding phylogenetic process, and affords a very fine instance of

the inheritance of acquired characters.

There are also many examples of this asymmetry of bilateral forms in

the plant world, such as the green foliage-leaves of the familiar

begonia and the blooms of _canna_.

IV. THE CENTRAPORIA.--Few organic forms are completely irregular and

without axes, as usually the attraction to the earth (geotaxis) or to

the nearest object determines the special direction of growth, and so

the formation of an axis in some direction or other. Nevertheless,

we may instance as quite irregular the soft and ever-changing

plasma-bodies of many rhizopods, the amœbinæ, mycetozoa, etc. Most

of the sponges also--which we regard as stocks of gastræads--are

completely irregular in structure; the most familiar example is the

common bath-sponge.

An impartial and thorough study of organic forms has convinced me that

their actual, infinitely varied configurations may all be reduced

to the few typical forms I have described. Comparative anatomy and

ontogeny further teach us that the countless modifying processes

which have led to the appearance of the various species have acted by

adaptation to different environments, habits, and customs, and give

us, in conjunction with heredity, a physiological explanation of this

morphological transformation. But the question arises as to the origin

of these few geometrically definable types, and the cause of their

divergence.

In this important and difficult question we find a great variety

of opinions and a strong leaning to dualistic and mystic theories.

Educated laymen, who have only a partial and imperfect acquaintance

with the biological facts, think that they are justified here in

appealing to a supernatural creation of forms. They contend that only a

wise creator, following a rational and conscious design, could produce

such structures. Even distinguished and informed scientists lean in

this matter towards mystic and transcendental ideas; they believe that

the ordinary natural forces do not suffice to explain these phenomena,

and that at least for the first construction of these fundamental types

we must postulate a deliberate creative thought, a design, or some such

teleological cause, and therefore consciously acting final causes. So

say Nägeli and Alexander Braun.

In direct opposition to this, I have ever maintained the view that the

action of familiar physical forces--mechanical efficient causes--fully

suffices to explain the origin and transformation of these fundamental

types, as well as for all other biological and inorganic processes. In

order to understand this monistic position thoroughly, and to meet the

errors of dualism, we must bear in mind always the radical processes of

growth which control all organic and inorganic configuration, and also

the long chain of advancing stages of development, which lead us from

the simplest protists, the monera, to the most advanced organisms.

The unicellular organisms exhibit the greatest variety from the

promorphological point of view. In the single class of the radiolaria

we find all imaginable geometrical types represented. This is seen

in a glance at the one hundred and forty plates on which I have

depicted thousands of these graceful little protozoa in my monograph

(_Challenger Report_, vol. xviii.). On the other hand, the monera, at

the lowest stage of organic life, the structureless organisms without

organs that live on the very frontier of the inorganic world, are very

simple. Especially interesting in this connection are the chromacea,

which have hitherto been so undeservedly and so incomprehensibly

neglected. Among the well-known and widely distributed chroococcacea,

the chroococcus, cœlosphærium, and aphanocapsa are quite the most

primitive of all organisms known to us--and at the same time the

organisms that enable us best to understand the origin of life by

spontaneous generation (archigony). The whole organism is merely a

tiny, bluish-green globule of plasm, without any structure, or only

surrounded by a thin membrane; its fundamental form is the simplest of

all, the centraxial smooth sphere. Next to these are the oscillaria

and nostochina, social chromacea, which have the appearance of thin,

bluish-green threads. They consist of simple primitive (unnucleated)

cells joined to each other; they seem often to be flattened into a

discoid shape as a result of close conjunction. Many protists are found

in two conditions, one mobile with very varied and changeable forms,

and one stationary with a globular shape. But when the separate living

cell begins to form a firm skeleton or protective cover for itself,

it may assume the most varied and often most complicated forms. In

this respect the class of the radiolaria among the protozoa, and the

class of the diatomes among the protophyta (both of which have flinty

shells), surpass all the other groups of the diversified realm of the

protists. In my _Art-forms in Nature_ I have given a selection of their

most beautiful forms (diatomes, A-f, 4, 84; radiolaria, A-f, 1, 11, 21,

22, 31, 41, 51, 61, 71, 95). The most remarkable and most important

fact about them is that the artistic builders of these wonderful and

often very ingenious and intricate flinty structures are merely the

plastidules or micella, the molecular and microscopically invisible

constituents of the soft viscous plasm (sarcode).

The configuration of the histona differs essentially from that of

the protists, since in the case of the latter the simple unicellular

body produces for itself alone the whole form and vital action of the

organism, while in the histona this is done by the cell state, or the

social combination of a number of different cells, which make up the

tissue body. Hence the ideal type which we can always define in the

actual histonal form has quite a different significance from that in

the unicellular protists. In the latter we find the utmost diversity in

the configuration of the independent living cells and the protective

cover it forms; among the histona the number of fundamental forms

is limited. It is true that the cells themselves which make up the

tissues may exhibit a great variety in form and structure; but the

number of the different tissues which they make up is small, and so

is the number of ideal types exhibited by the organism they combine

to form--the sprout (_culmus_) in the plant kingdom and the person in

the animal kingdom. The same may be said of the stock (_cormus_) in

both kingdoms--that is to say, of the higher individual unity which is

constituted by the union of several sprouts or persons.

The two classes of fundamental forms which are especially found in the

plant sprouts or the animal persons are the radial and bilateral. The

one is determined by the stationary life, the other by free movement in

a certain attitude and direction (swimming in water or creeping on the

ground). Hence we find the radial form (as pyramidal) predominant in

the blooms and fruits of the metaphyta, and the persons of the polyps,

corals, and regular echinoderms. On the other hand, the bilateral or

dorsiventral form preponderates in most free-moving animals; though

it is also found in many flowers (papilionaceous and labial flowers,

orchids, and others that are fertilized by insects). Here we have

to seek the cause of the bilateralism in different features, in the

relations with the insects, in the mode of their fastening to and

distribution on the stalk (for the green foliage leaves), and so on.

The complex individuals of the first order, the stocks (_cormi_), are

more dependent in their growth on the spatial conditions of their

environment than the sprouts or persons; hence their typical form is

generally more or less irregular, and rarely bilateral.

The interest which we take in natural and artistic forms, and which

has for thousands of years prompted men to reproduce the former in

the latter, depends for the most part, if not altogether, on their

beauty--that is to say, on the feeling of pleasure we experience in

looking at them. The causes of this pleasure and joy in the beautiful

and the naturalness of its development are explained in æsthetics. When

we combine this science with the results of modern cerebral physiology,

we may distinguish two classes of beauty--direct and indirect. In

direct or sensible beauty the internal sense-organs, or the æsthetic

neurona or sense-cells of the brain, are immediately affected with

pleasure. But in indirect or associational beauty these impressions

are combined with an excitement of the phronetic neurona--the rational

brain-cells which effect presentation and thought.

Direct or sensible beauty (the subject of sensual æsthetics) is the

direct perception of agreeable stimuli by the sense-organs. We may

distinguish the following stages of its perfection: 1. Simple beauty

(the subject of primordial æsthetics); the pleasure is evoked by the

direct sense-impression of a simple form or color. Thus, for instance,

a wooden sphere makes an agreeable impression as compared with a

shapeless piece of wood, a crystal as compared with a stone, a sky-blue

or golden-yellow spot as compared with a greenish-blue or dull-yellow

one (in music a simple pure bell-tone as compared with a shrill

whistle). 2. Rhythmic beauty (the subject of linear æsthetics); the

æsthetic sensation is caused by the serial repetition of some simple

form--for instance, a pearl necklace, a chainlike community of monera

(nostoc) or of cells (diatomes, A-f, 84, figs. 7 and 9): in music a

tasteful series of simple notes. 3. Actinal beauty (the subject of

radial æsthetics); the pleasure is excited by the orderly arrangement

of three or more homogeneous simple forms about a common centre, from

which they radiate; for instance, a regular cross or a radiating

star, the three counter-pieces in the iris-bloom, the four paramera

in the body of the medusa, the five radial-pieces in the star-fish.

The familiar experience of the kaleidoscope shows how amply the simple

radial constellation of three or more simple figures may delight

our æsthetic sense (in music we have the simple harmony of several

simultaneous notes). 4. Symmetrical beauty (the subject of bilateral

æsthetics); the pleasure is caused by the relation of a simple object

to its like, the mutual completion of two similar halves (the right and

left parts). When we fold a piece of paper over an ink-stain in such a

way that it is equally impressed on both halves of the fold, we get a

symmetrical figure which makes an agreeable impression on our natural

sense of space or equilibrium.

The æsthetic impressions in indirect associational beauty (the subject

of associative or symbolical æsthetics) are not only much more varied

and complex than those we have described, but they also play a much

more important part in the life of man and the higher animals. The

anatomic condition for this higher physiological function is the

elaborate construction of the brain in the higher animals and man,

and particularly the development of the special association-centres

(thought-centres, reason-sphere) and their differentiation from the

internal sense-centres. In this millions of different neurona or

psychic cells co-operate, the sensual æstheta acting in conjunction

with the rational phroneta, and thus, by complex associations of

ideas, much higher and more valuable functions arise. We may indicate

four chief groups of this associational or indirect beauty. 5.

Biological beauty (the subject of botanical and zoological æsthetics):

the various forms of organisms and their organs (for instance, a

flower, a butterfly) excite our æsthetic interest by association with

their physiological significance, their movements, their bionomic

relations, their practical use, and so on. 6. Anthropistic beauty

(the subject of anthropomorphic æsthetics): man, as "the measure of

all things," regards his own organism as the chief object of beauty,

either morphologically considered (beauty of the whole body and

its various organs--the eyes, mouth, hair, flesh-tint, etc.), or

physiologically (beauty of movements or positions), or psychologically

(the expression of the emotions in the physiognomy). As man transfers

to the objective world this personal gratification he experiences from

self-consideration, and anthropomorphically regards other beings in

the light of them, this anthropistic æsthetic obtains a far-reaching

significance. 7. Sexual beauty (the subject of erotic æsthetics): the

pleasure is caused by the mutual attraction of the sexes. The supreme

importance of love in the life of man and most other organisms,

the powerful influence of the passions, the sexual selection that

is associated with reproduction, have evoked an infinite number of

æsthetic creations in every branch of art relating to the antithesis of

man and woman. The special pleasure which is caused by the bodily and

mental affinities of the sexes can be traced phylogenetically to the

cell-love of the two sexual cells, or the attraction of the sperm-cell

to ovum. 8. Landscape beauty (the subject of regional æsthetics): the

pleasure which is caused by the sight of a fine landscape, and that

finds satisfaction in modern landscape-painting, is more comprehensive

than that of any other æsthetic sensations. In point of space the

object is larger and richer than any of the individual objects in

nature which are beautiful and interesting in themselves. The varying

forms of the clouds and the water, the outline of the blue mountains

in the background, the woods and meadows in the middle-distance,

and the living figures in the foreground, excite in the mind of the

spectator a number of different impressions which are woven together

into a harmonious whole by a most elaborate association of ideas. The

physiological functions of the nerve-cells in the cortex which effect

these æsthetic pleasures, and the interaction of the sensual æstheta

with the rational phroneta, are among the most perfect achievements

of organic life. This "regional æsthetics," which has to establish

scientifically the laws of landscape beauty, is much younger than the

other branches of the science of the beautiful. It is very remarkable

that absolute irregularity, the absence of symmetry and mathematical

forms, is the first condition for the beauty of a landscape (as

contrasted with architecture, and the beauty of separate objects in

nature). Symmetrical arrangement of things (such as a double row of

poplars or houses) or radial figures (a flower-bed or artificial wood)

do not please the finer taste for landscape; they seem tedious.

A comparative survey of these eight kinds of beauty in natural forms

discovers a connected development, rising from the simple to the

complex, from the lower to the higher. This scale corresponds to the

evolution of the sense of beauty in man, ontogenetically from the

child to the adult, phylogenetically from the savage to the civilized

man and the art critic. The stem-history of man and his organs, which

explains to us in anthropogeny the gradual rise from lower to higher

forms by the interaction of heredity and adaptation, also finds an

application in the history of æsthetics and ornamentation. It teaches

us how feeling, taste, emotion, and art have been gradually evolved. On

the other hand, we have corresponding to this evolutionary series the

scale of the typical forms which lie at the root of the real forms of

bodies both in nature and art.

SEVENTH TABLE

THE MORPHOLOGICAL SYSTEM OF ORGANISMS

Division of living things (plants and animals) into two kingdoms

(protista and histona) on the ground of their cell-structure and

body-structure.

┌──────────────────────────────────────────────────────────┐

│ First organic kingdom: UNICELLULAR, protista. │

│ │

│Organisms which as a rule remain unicellular │

│throughoutlife (_monobia_), less frequently they │

│form loose cell communities (_cœnobia_) by │

│repeated cleavage, but never real tissues. │

│ │

│ Sub─kingdom of the protista. │

├───────────────────────────┬──────────────────────────────┤

│ A. PRIMITIVE PLANTS │ B. PRIMITIVE ANIMALS │

│ (protophyta). │ (protozoa). │

│ │ │

│ A. Character: │ B. Character: │

│ Plasmodomous. │ Plasmophagous. │

│ │ │

│ Unicellulars with │ Unicellulars with │

│ vegetal metabolism: │ animal metabolism: │

│ Carbon─assimilation. │ Albumin─assimilation. │

│ │ │

│ CHIEF GROUPS: │ CHIEF GROUPS: │

│ │ │

│ I. Phytomonera │ I. Zoomonera. │

│ │ │

│Protophyta without nucleus │ Protozoa without nucleus │

│ (monera) │ (monera). │

│ Chromacea │ Bacteria. │

│ │ │

│ II. Algariæ. │ II. Sporozoa. │

│ │ │

│Unicellular algæ with │ Nucleated protozoa without │

│nucleus, without ciliary │ mobile processes: Gregarinæ, │

│motion: Paulotomea, │ chytridinæ. │

│diatomea. │ │

│ │ │

│ III. Algettæ. │ III. Rhizopoda. │

│ │ │

│Unicellular algæ with │ Nucleated protozoa with │

│nucleus, and with ciliary │ pseudopodia: Labosa, │

│motion: Mastigota, │ radiolaria. │

│melthallia, siphonea. │ │

│ │ IV. Infusoria. │

│ │ │

│ │ Nucleated protozoa with │

│ │ cilia or lashes: │

│ │ Flagellata, ciliata. │

└───────────────────────────┴──────────────────────────────┘

┌────────────────────────────────────────────────────────────┐

│ Second organic kingdom: MULTICELLULAR, histona. │

│ │

│ Organisms which are only unicellular at the │

│ beginning of their existence, are later │

│ multicellular, and always form real tissues │

│ (_histobia_) by the firm conjunction of social cells. │

│ │

│ Sub─kingdom of the histona. │

├────────────────────────────┬───────────────────────────────┤

│ C. TISSUE PLANTS │ D. TISSUE ANIMALS │

│ (metaphyta). │ (metazoa). │

│ │ │

│ C. Character: │ D. Character: │

│ Plasmodomous. │ Phasmophagous. │

│ │ │

│ Multicellulars with │ Multicellulars with │

│ vegetal metabolism: │ animal metabolism: │

│ Carbon─assimilation. │ Albumin─assimilation. │

│ │ │

│ CHIEF GROUPS: │ CHIEF GROUPS: │

│ │ │

│ I. Thallophyta. │ I. Cœlenteria │

│ │ (cœlenterata). │

│ Thallus─plants. Metaphyta │ │

│ with thallus: Algæ, mycetæ │ Metazoa without body │

│ (fungi). │ cavity and anus: Gastræada. │

│ │ Sponges, cnidaria, platodes. │

│ II. Mesophyta. │ │

│ │ II. Cœlomaria │

│ Median plants, with │ (bilaterals). │

│ prothallium: Mosses, ferns │ │

│ (muscinæ filicinæ). │ Metazoa with body cavity │

│ │ and anus (generally also │

│ │ blood─vessels). Vermalia, │

│ III. Anthophyta │ mollusca, echinoderma, │

│ (phanerogams). │ articulata, tunicata, │

│ │ vertebrata. │

│ Flowering plants, with │ │

│ blooms and seeds │ │

│ (spermophyta): │ │

│ Gymnosperms, angiosperms. │ │

│ │ │

│ │ │

│ │ │

│ │ │

└────────────────────────────┴───────────────────────────────┘

IX

MONERA

The simplest forms of life--Cell theory and cell dogma--Precellular

organisms: monera, cytodes, and cells--Actual monera--Chromacea

(cyanophyceæ)--Chromatophora--Cœnobia of chromacea: vital

phenomena--Bacteria--Relations of the bacteria to the chromacea,

the fungi, and the protozoa--Rhizomonera (protamœba, protogenes,

protomyxa, bathybius)--Problematic monera--Phytomonera (plasmodoma)

and zoomonera (plasmophaga)--Transition between the two classes.

In the study and explanation of all complex phenomena the first

thing to do is to understand the simple parts, the manner of their

combination, and the development of the compound from the simple. This

principle applies generally to inorganic objects, such as minerals,

artificially constructed machines, etc. It is also of general

application in biological work. The efforts of comparative anatomy are

directed to the comprehension of the intricate structure of the higher

organisms from the rising scale of organization and life in the lower,

and the origin of the former by historical development from the latter.

The modern science of the cell (cytology), which has in a short time

attained a considerable rank, pursues a method in opposition to this

principle. The intricate composition of the unicellular organism, in

many of the higher protists (such as the ciliata and infusoria) and

many of the higher tissue-cells (such as the neurona) has led to the

erroneous ascription of a highly complex organization to the cell in

general. One would be justified in saying that of late the cell-theory

has established itself in the dangerous and misleading position of a

cell-dogma.

The modern treatment of the science, as we find it in numbers of recent

works, even in some of the most distinguished manuals, and which we

must resent on account of its dogmatism, culminates in something like

the following theses:

1. The nucleated cell is the general elementary organism; all living

things are either unicellular, or made up of a number of cells and

tissues.

2. This elementary organism consists of at least two different organs

(or, more correctly, organella), the internal nucleus and the outer

cell-body (or cytoplasm).

3. The matter in each of these cell-organs--the caryoplasm of the

nucleus and the cytoplasm of the body--is never homogeneous (or

consisting of a chemical substratum), but always "organized," or

made up of several chemically and anatomically different elementary

constituents.

4. The plasm (or protoplasm) is, therefore, a morphological, not a

chemical, unity.

5. Every cell comes (and has come) only from a mother-cell, and every

nucleus from a mother-nucleus (_omnis cellula e cellula--omnis nucleus

e nucleo_).