PART I

THE INDIVIDUAL ORGANISM WITH REGARD TO FORM AND METABOLISM

*A.* ELEMENTARY MORPHOGENESIS

EVOLUTIO AND EPIGENESIS IN THE OLD SENSE

The organism is a specific body, built up by a typical combination

of specific and different parts. It is implied in the words of this

definition, that the organism is different, not only from crystals, as

was mentioned in the last lecture, but also from all combinations of

crystals, such as those called dendrites and others, which consist of a

typical arrangement of identical units, the nature of their combination

depending on the forces of every single one of their parts. For this

reason dendrites, in spite of the typical features in their combination,

must be called aggregates; but the organism is not an aggregate even

from the most superficial point of view.

We have said before, what must have been familiar to you already, that

the organism is not always the same in its individual life, that it

has its development, leading from simpler to more complicated forms of

combination of parts; there is a “production of visible manifoldness”

carried out during development, to describe the chief character of that

process in the words of Wilhelm Roux. We leave it an open question in

our present merely descriptive analysis, whether there was already a

“manifoldness,” in an invisible state, before development, or whether

the phrase “production of manifoldness” is to be understood in an

absolute sense.

It has not always been granted in the history of biology, and of

embryology especially, that production of visible manifoldness is the

chief feature of what is called an organism’s embryology or ontogeny:

the eighteenth century is full of determined scientific battles over the

question. One school, with Albert von Haller and Bonnet as its leading

men, maintained the view that there was no production of different parts

at all in development, this process being a mere “evolutio,” that is,

a growth of parts already existing from the beginning, yes, from the

very beginning of life; whilst the other school, with C. F. Wolff and

Blumenbach at its head, supported the opposite doctrine of so-called

“epigenesis,” which has been proved to be the right one.

To some extent these differences of opinion were only the outcome of the

rather imperfect state of the optical instruments of that period. But

there were also deeper reasons beyond mere difficulties of description;

there were theoretical convictions underlying them. It is *impossible*,

said the one party, that there is any real production of new parts;

there *must* be such a production, said the other.

We ourselves shall have to deal with these questions of the theory of

organic development; but at present our object is narrower, and merely

descriptive. It certainly is of great importance to understand most

clearly that there actually *is* a “production of visible manifoldness”

during ontogenesis in the descriptive sense; the knowledge of the fact

of this process must be the very foundation of all studies on the theory

of development in any case, and therefore we shall devote this whole

lecture to studies in merely descriptive embryology.

But descriptive embryology, even if it is to serve merely as an

instance of the universality of the fact of epigenesis, can only be

studied successfully with reference to a concrete case. We select the

development of the common sea-urchin (*Echinus microtuberculatus*) as

such a case, and we are the more entitled to select this organism rather

than another, because most of the analytical experimental work, carried

out in the interests of a real theory of development, has been done

on the germs of this animal. Therefore, to know at least the outlines

of the individual embryology of the Echinus may indeed be called the

*conditio sine qua non* for a real understanding of what is to follow.

THE CELL[3]

[3] E. B. Wilson, *The Cell in Development and Inheritance*, New York,

Macmillan, 1896.

You are aware that all organisms consist of organs and that each of

their organs has a different function: the brain, the liver, the eyes,

the hands are types of organs in animals, as are the leaves and the

pistils in plants.

You are also aware that, except in the lowest organisms, the so-called

Protista, all organs are built up of cells. That is a simple fact of

observation, and I therefore cannot agree with the common habit of

giving to this plain fact the title of cell-“theory.” There is nothing

theoretical in it; and, on the other hand, all attempts to conceive

the organism as a mere aggregate of cells have proved to be wrong. It

is *the whole* that uses the cells, as we shall see later on, or that

may not use them: thus there is nothing like a “cell-theory,” even in a

deeper meaning of the word.

The cell may have the most different forms: take a cell of the skin, of

a muscle, of a gland, of the wood in plants as typical examples. But in

every case two parts may be distinguished in a cell: an outside part,

the protoplasm, and an inside part, the nucleus, to leave out of special

account several others, which, by the way, may only be protoplasmatic

modifications.

Protoplasm is a mere name for what is not the nucleus; in any case it is

not a homogeneous chemical compound; it consists of many such compounds

and has a sort of architecture; all organic functions are based upon

its metabolism. The nucleus has a very typical structure, which stands

in a close relation to its behaviour during the most characteristic

morphological period of the cell: during its division. Let us devote a

few words to a consideration of this division and the part the nucleus

plays in it; it will directly bear on future theoretical considerations

about development.

There is a certain substance in every nucleus of a cell which stains

most markedly, whenever cells are treated with pigments: the name

of “chromatin” has been given to it. The chromatin always gives the

reaction of an acid, while protoplasm is basic; besides that it seems to

be a centre of oxidation. Now, when a division of a cell is to occur,

the chromatin, which had been diffusely distributed before, in the form

of small grains, arranges itself into a long and very much twisted

thread. This thread breaks, as it were by sections, into almost equal

parts, typical in number for each species, and each of these parts is

split at full length. A certain number of pairs of small threads, the

so-called “chromosomes,” are the ultimate result of this process, which

intentionally has been described a little schematically, the breaking

and the splitting in fact going on simultaneously or occasionally even

in reverse order. While what we have described is performing in the

nucleus, there have happened some typical modifications in protoplasm,

and then, by an interaction of protoplasmatic and nuclear factors, the

first step in the actual division of the cell begins. Of each pair of

the small threads of chromatin one constituent is moved to one side of

the cell, one to the other; two daughter-nuclei are formed in this way;

the protoplasm itself at the same time forms a circular furrow between

them; the furrow gets deeper and deeper; at last it cuts the cell in

two, and the division of the cell is accomplished.

Not only is the growth of the already typically formed organism carried

out by a series of cell-divisions, but also development proper in our

sense, as a “production of visible manifoldness,” is realised to a great

extent by the aid of such divisions, which therefore may indeed be said

to be of very fundamental importance (Fig. 1).

[Illustration: Fig. 1.--Diagram of Cell-Division (*after* Boveri).

*a.* Resting cell; the chromatin distributed in the form of small

granules inside the nucleus. Outside the nucleus is the “centrosome,”

not mentioned in the text.

*b.* Beginning of division; the chromatin arranged in the form of a long

thread. Centrosome divided in two.

*c.* The thread of chromatin cut into four parts, the “chromosomes.”

*d.* The four parts of the chromatin arranged symmetrically between the

centrosomes and the star-like “spheres.”

*e.* Each of the chromosomes split at full length.

*f.* Beginning of division of protoplasm; the two parts of each

chromosome separated.

*g.* End of cell-division.]

Each cell-division which promotes growth is followed by the

enlargement of the two daughter-cells which result from it; these two

daughter-elements attain the exact size of the mother-cell before

division, and as soon as this size is reached a new division begins:

so the growth of the whole is in the main the result of the growth of

the elements. Cell-divisions during real organ-formation may behave

differently, as will be described at a proper occasion.

THE EGG: ITS MATURATION AND FERTILISATION

We know that all the organs of an animal or plant consist of cells, and

we know what acts a cell can perform. Now there is one very important

organ in all living beings, which is devoted to reproduction. This

organ, the so-called ovary in animals, is also built up of cells,

and its single cells are called the eggs; the eggs originated by

cell-division, and cell-division is to lead from them to the new adult.

But, with a very few exceptions, the egg in the ovary is not able to

accomplish its functions, unless certain typical events have occurred,

some of which are of a merely preparatory kind, whilst the others are

the actual stimulus for development.

The preparatory ones are generally known under the name of “maturation.”

The egg must be “mature,” in order that it may begin development, or

even that it may be stimulated to it. Maturation consists of a rather

complicated series of phenomena: later on we shall have occasion to

mention, at least shortly, what happens in the protoplasm during its

course; as to the nuclear changes during maturation it may be enough

for our purposes to say, that there occur certain processes among the

chromosomes, which lead to an extension of half of them in the form

of two very small cells, the “directive cells” or “directive or polar

bodies,” as they have been somewhat cautiously called.

The ripe or mature egg is capable of being fertilised.

Before turning to this important fact, which, by the way, will bring us

to our specially chosen type, the Echinus, a few words may be devoted to

the phenomenon of “parthenogenesis,” that is to say, the possibility

of development without fertilisation, since owing to the brilliant

discoveries of the American physiologist, Jacques Loeb, this topic forms

one of the centres of biological interest at present. It has long been

known that the eggs of certain bees, lice, crayfishes, and other animals

and also plants, are capable of development without fertilisation at

all. Now Richard Hertwig and T. H. Morgan already had shown, that at

least nuclear division may occur in the eggs of other forms--in the egg

of the sea-urchin for instance--when these eggs are exposed to some

chemical injuries. But Loeb[4] succeeded in obtaining a full development

by treating the eggs of echinoderms with chloride of magnesium; thus

artificial parthenogenesis had been discovered. Later researches have

shown that artificial parthenogenesis may occur in all classes of the

animal kingdom and may be provoked by all sorts of chemical or physical

means. We do not know at present in what the proper stimulus consists

that must be supposed here to take the place of fertilisation; it seems,

of course, highly probable that it is always the same in the last

resort.[5]

[4] *Amer. Journ. Physiol.* vols. iii. and iv. 1900.

[5] According to Delage (*Arch. Zool. exp.*, 3 sér. 10, 1902), it is

indifferent for the realisation of artificial parthenogenesis, whether

but one, or both, or neither of the “polar bodies” has been formed. But

the egg must be in the first stages of maturation to the extent that the

“nuclear membrane” must be already dissolved.

But enough about processes, which at present are of a highly scientific,

but hardly of any philosophic interest.

By fertilisation proper we understand the joining of the male element,

the spermatozoon or the spermia, with the female element, the egg. Like

the egg, the spermatozoon is but a cell, though the two differ very much

from one another in the relation between their protoplasm and nucleus:

in all eggs it is the protoplasm which is comparatively very large, if

held together with somatic cells, in the spermatozoon it is the nucleus.

A large amount of reserve material, destined for the growth of the

future being, is the chief cause of the size of the egg-protoplasm. The

egg is quite or almost devoid of the faculty of movement, while on the

contrary, movement is the most typical feature of the spermia. Its whole

organisation is adapted to movement in the most characteristic manner:

indeed, most spermatozoa resemble a swimming infusorium, of the type

of Flagellata, a so-called head and a moving tail are their two chief

constituents; the head is formed almost entirely of nuclear substance.

It seems that in most cases the spermatozoa swim around at random and

that their union with the eggs is assured only by their enormous number;

only in a few cases in plants have there been discovered special stimuli

of a chemical nature, which attract the spermia to the egg.

But we cannot enter here more fully into the physiology of

fertilisation, and shall only remark that its real significance is by no

means clear.[6]

[6] The older theories, attributing to fertilisation (or to

“conjugation,” *i.e.* its equivalent in Protozoa), some sort of

“renovation” or “rejuvenescence” of the race, have been almost

completely given up. (See Calkins, *Arch. für Entwickelungsmechanik*,

xv. 1902). R. Hertwig recently has advocated the view, that abnormal

relations between the amounts of nuclear and of protoplasmatic material

are rectified in some way by those processes. Teleologically, sexual

reproduction has been considered as a means of variability (Weismann),

but also as a means of preserving the type!

THE FIRST DEVELOPMENT PROCESS OF ECHINUS

Turning now definitively to the special kind of organism, chosen of our

type, the common sea-urchin, we properly begin with a few words about

the absolute size of its eggs and spermatozoa. All of you are familiar

with the eggs of birds and possibly of frogs; these are abnormally

large eggs, on account of the very high amount of reserve material they

contain. The almost spherical egg of our Echinus only measures about a

tenth of a millimetre in diameter; and the head of the spermatozoon has

a volume which is only the four-hundred-thousandth part of the volume

of the egg! The egg is about on the extreme limit of what can be seen

without optical instruments; it is visible as a small white point. But

the number of eggs produced by a single female is enormous and may

amount to hundreds of thousands; this is one of the properties which

render the eggs of Echinus so very suitable for experimental research;

you can obtain them whenever and in any quantity you like; and,

moreover, they happen to be very clear and transparent, even in later

stages, and to bear all kinds of operations well.

The spermia enters the egg, and it does so in the open water--another

of the experimental advantages of our type. Only one spermia enters

the egg in normal cases, and only its head goes in, the tail is left

outside. The moment that the head has penetrated the protoplasm of the

egg a thin membrane is formed by the latter. This membrane is very soft

at first, becoming much stronger later on; it is very important for all

experimental work, that by shaking the egg in the first minutes of its

existence the membrane can easily be destroyed without any damage to the

egg itself.

And now occurs the chief phenomenon of fertilisation: the nucleus of

the spermatozoon unites with the nucleus of the egg. When speaking of

maturation, we mentioned that half of the chromatin was thrown out of

the egg by that process: now this half is brought in again, but comes

from another individual.

It is from this phenomenon of nuclear union as the main character of

fertilisation that almost all theories of heredity assume their right to

regard the nuclei of the sexual cells as the true “seat” of inheritance.

Later on we shall have occasion to discuss this hypothesis from the

point of view of logic and fact.

After the complete union of what are called the male and the female

“pronuclei,” the egg begins its development; and this development, in

its first steps, is simply pure cell-division. We know already the chief

points of this process, and need only add to what has been described,

that in the whole first series of the cell-divisions of the egg, or,

to use the technical term, in the whole process of the “cleavage” or

“segmentation” of it, there is never any growth of the daughter-elements

after each division, such as we know to occur after all cell-divisions

of later embryological stages. So it happens, that during cleavage the

embryonic cells become smaller and smaller, until a certain limit is

reached; the sum of the volumes of all the cleavage cells together is

equal to the volume of the egg.

But our future studies will require a more thorough knowledge of the

cleavage of our Echinus; the experimental data we shall have to describe

later on could hardly be properly understood without such knowledge.

The first division plane, or, as we shall say, the first cleavage

plane, divides the eggs into equal parts; the second lies at right

angles to the first and again divides equally: we now have a ring of

four cells. The third cleavage plane stands at right angles to the

first two; it may be called an equatorial plane, if we compare the egg

with a globe; it also divides equally, and so we now find two rings,

each consisting of four cells, and one above the other. But now the

cell-divisions cease to be equal, at least in one part of the egg: the

next division, which leads from the eight- to the sixteen-cell stage of

cleavage, forms four rings, of four cells each, out of the two rings of

the eight-cell stage. Only in one half of the germ, in which we shall

call the upper one, or which we might call, in comparison with a globe,

the northern hemisphere, are cells of equal size to be found; in the

lower half of the egg four very small cells have been formed at one

“pole” of the whole germ. We call these cells the “micromeres,” that

is, the “small parts,” on the analogy of the term “blastomeres,” that

is, parts of the germ, which is applied to all the cleavage cells in

general. The place occupied by the micromeres is of great importance

to the germ as a whole: the first formation of real organs will start

from this point later on. It is sufficient thus fully to have studied

the cleavage of our Echinus up to this stage: the later cleavage stages

may be mentioned more shortly. All the following divisions are into

equal parts; there are no other micromeres formed, though, of course,

the cells derived from the micromeres of the sixteen-cell stage always

remain smaller than the rest. All the divisions are tangential; radial

cleavages never occur, and therefore the process of cleavage ends at

last in the formation of one layer of cells, which forms the surface

of a sphere; it is especially by the rounding-up of each blastomere,

after its individual appearance, that this real surface layer of cells

is formed, but, of course, the condition, that no radial divisions

occur, is the most important one in its formation. When 808 blastomeres

have come into existence the process of cleavage is finished; a sphere

with a wall of cells and an empty interior is the result. That only 808

cells are formed, and not, as might be expected, 1024, is due to the

fact that the micromeres divide less often than the other elements; but

speaking roughly, of course, we may say that there are ten steps of

cleavage-divisions in our form; 1024 being equal to 2^{10}.

We have learned that the first process of development, the cleavage, is

carried out by simple cell-division. A few cases are known, in which

cell-division during cleavage is accompanied by a specific migration of

parts of the protoplasm in the interior of the blastomeres, especially

in the first two or first four; but in almost all instances cleavage

is as simple a process of mere division as it is in our sea-urchin.

Now the second step in development, at least in our form, is a typical

histological performance: it gives a new histological feature to all

of the blastomeres: they acquire small cilia on their outer side and

with these cilia the young germ is able to swim about after it has left

its membrane. The germ may be called a “blastula” at this stage, as it

was first called by Haeckel, whose useful denominations of the first

embryonic stages may conveniently be applied, even if one does not agree

with most, or perhaps almost all, of his speculations (Fig. 2).

[Illustration: Fig. 2.--Early Development of Echinus, the Common

Sea-urchin.

*a.* Two cells.

*b.* Four cells.

*c.* Eight cells, arranged in two rings of four, above one another.

*d.* Sixteen cells, four “micromeres” formed at the “vegetative” pole.

*e.* Optical section of the “blastula,” a hollow sphere consisting of

about one thousand cells, each of them with a small cilium.]

It is important to notice that the formation of the “blastula” from the

last cleavage stage is certainly a process of organisation, and may also

be called a differentiation with regard to that stage. But there is in

the blastula no trace of one *part* of the germ becoming different with

respect to others of its parts. If development were to go on in this

direction alone, high organisatory complications might occur: but there

would always be only one sort of cells, arranged in a sphere; there

would be only one kind of what is called “tissue.”

But in fact development very soon loads to true differences of the

parts of the germ with respect to one another, and the next step of the

process will enable us to apply different denominations to the different

parts of the embryo.

At one pole of the swimming blastula, exactly at the point where the

descendants of the micromeres are situated, about fifty cells lose

contact with their neighbours and leave the surface of the globe, being

driven into the interior space of it. Not very much is known about the

exact manner in which these changes of cellular arrangement are carried

out, whether the cells are passively pressed by their neighbours, or

whether, perhaps, in a more active manner, they change their surface

conditions; therefore, as in most ontogenetic processes, the description

had best be made cautiously in fairly neutral or figurative words.

The cells which in the above manner have entered the interior of the

blastula are to be the foundation of important parts of the future

organism; they are to form its connective tissue, many of its muscles,

and the skeleton. “Mesenchyme,” *i.e.* “what has been infused into the

other parts,” is the technical name usually applied to these cells.

We now have to learn their definite arrangement. At first they lie

as a sort of heap inside the cell wall of the blastula, inside the

“blastoderm,” *i.e.* skin of the germ. But soon they move from one

another, to form a ring round the pole at which they entered, and on

this ring a process takes place which has a very important bearing upon

the whole type of the organisation of the germ. You will have noticed

that hitherto the germ with regard to its symmetry has been a monaxial

or radial formation; the cleavage stages and the blastula with its

mesenchyme were forms with two different poles, lying at the ends of one

single line, and round this line everything was arranged concentrically.

But now what is called “bilateral symmetry” is established; the

mesenchyme ring assumes a structure which can be symmetrically divided

only by one plane, but divided in such a way, that one-half of it is the

mirror image of the other. A figure shows best what has occurred, and

you will notice (Fig. 3) two masses of cells in this figure, which have

the forms of spherical triangles: it is in the midst of these triangles

that the skeleton of the larva *originates*. The germ had an upper and

a lower side before: it now has got an upper and lower, front and back,

*right and left* half; it now has acquired that symmetry of organisation

which our own body has; at least it has got it as far as its mesenchyme

is concerned.

[Illustration: Fig. 3.--Formation of Mesenchyme in Echinus.

*a.* Outlines of blastula, side-view; mesenchyme forms a heap of cells

at the “vegetative” pole.

*a*_1. Heap of mesenchyme-cells from above.

*b.* Mesenchyme-cells arranged in a ring round the vegetative pole.

*c.* Mesenchyme-cells arranged in a bilateral-symmetrical figure;

primordia of skeleton in the midst of two spherical triangles.]

We leave the mesenchyme for a while and study another kind of

organogenesis. At the very same pole of the germ where the mesenchyme

cells originated there is a long and narrow tube of cell growing in,

and this tube, getting longer and longer, after a few hours of growth

touches the opposite pole of the larva. The growth of this cellular tube

marks the beginning of the formation of the intestine, with all that

is to be derived from it. The larva now is no longer a blastula, but

receives the name of “gastrula” in Haeckel’s terminology; it is built

up of the three “germ-layers” in this stage. The remaining part of the

blastoderm is called “ectoderm,” or outer layer; the newly-formed tube,

“endoderm,” or inner layer; while the third layer is the “mesenchyme”

already known to us.

The endoderm itself is a radial structure at first, as was the whole

germ in a former stage, but soon its free end bends and moves against

one of the sides of the ectoderm, against that side of it where the two

triangles of the mesenchyme are to be found also. Thus the endoderm

has acquired bilateral symmetry just as the mesenchyme before, and

as in this stage the ectoderm also assumes a bilateral symmetry

in its form, corresponding with the symmetrical relations in the

endoderm and the mesenchyme, we now may call the whole of our larva a

bilateral-symmetrical organisation.

It cannot be our task to follow all the points of organogenesis of

Echinus in detail. It must suffice to state briefly that ere long a

second portion of the mesenchyme is formed in the larva, starting from

the free end of its intestine tube; that the formation of the so-called

“coelum” occurs by a sort of splitting off from this same original

organ; and that the intestine itself is divided into three parts of

different size and aspect by two circular sections.

But we must not, I think, dismiss the formation of the skeleton so

quickly. I told you already that the skeleton has its first origin in

the midst of the two triangular cell-masses of the mesenchyme; but what

are the steps before it attains its typical and complicated structure?

At the beginning a very small tetrahedron, consisting of carbonate of

calcium, is formed in each of the triangles; the four edges of the

tetrahedron are produced into thin rods, and by means of a different

organogenesis along each of these rods the typical formation of the

skeleton proceeds. But the manner in which it is carried out is very

strange and peculiar. About thirty of the mesenchyme cells are occupied

in the formation of skeleton substance on each side of the larva. They

wander through the interior space of the gastrula--which at this stage

is not filled with sea water but with a sort of gelatinous material--and

wander in such a manner that they always come to the right places, where

a part of the skeleton is to be formed; they form it by a process of

secretion, quite unknown in detail; one of them forms one part, one the

other, but what they form altogether, is one whole.

When the formation of the skeleton is accomplished, the typical larva

of our Echinus is built up; it is called the “pluteus” (Fig. 4). Though

it is far from being the perfect adult animal, it has an independent

life of its own; it feeds and moves about and does not go through any

important changes of form for weeks. But after a certain period of

this species of independent life as a “larva,” the changes of form it

undergoes again are most fundamental: it must be transformed into the

adult sea-urchin, as all of you know. There are hundreds and hundreds

of single operations of organogenesis to be accomplished before that

end is reached; and perhaps the strangest of all these operations is a

certain sort of growth, by which the symmetry of the animal, at least

in certain of its parts--not in all of them--is changed again from

bilateral to radial, just the opposite of what happened in the very

early stages.

[Illustration: Fig. 4.--Larval Development of Echinus.

*A.* The gastrula.

*B.* Later stage, bilateral-symmetrical. Intestine begins to divide

into three parts.

*C.* Pluteus larva. S = Skeleton. I = Intestine.]

But we cannot follow the embryology of our Echinus further here; and

indeed we are the less obliged to do so, since in all our experimental

work we shall have to deal with it only as far as to the pluteus larva.

It is impossible under ordinary conditions to rear the germs up to the

adult stages in captivity.

You now, I hope, will have a general idea at least of the processes

of which the individual development of an animal consists. Of course

the specific features leading from the egg to the adult are different

in each specific case, and, in order to make this point as clear as

possible, I shall now add to our description a few words about what may

be called a comparative descriptive embryology.

COMPARATIVE EMBRYOLOGY

Even the cleavage may present rather different aspects. There may be a

compact blastula, not one surrounded by only one layer of cells as in

Echinus; or bilaterality may be established as early as the cleavage

stage--as in many worms and in ascidians--and not so late as in Echinus.

The formation of the germ layers may go on in a different order and

under very different conditions: a rather close relative of our Echinus,

for instance, the starfish, forms first the endoderm and afterwards

the mesenchyme. In many cases there is no tube of cells forming the

“endoderm,” but a flat layer of cells is the first foundation of all the

intestinal organs: so it is in all birds and in the cuttlefish. And,

as all of you know, of course, there are very many animal forms which

have no proper “larval” stage: there is one in the frog, the well-known

“tadpole,” but the birds and mammals have no larvae; that is to say,

there is no special stage in the ontogeny of these forms which leads an

independent life for a certain time, as if it were a species by itself,

but all the ontogenetical stages are properly “embryonic”--the germ is

always an “embryo” until it becomes the perfect young organism. And you

also know that not all skeletons consist of carbonate of calcium, but,

that there are skeletons of silicates, as in Radiolaria, and of horny

substance, as in many sponges. And, indeed, if we were to glance at the

development of plants also, the differences would seem to us probably

so great that all the similarities would seem to disappear.

But there are similarities, nevertheless, in all development, and we

shall now proceed to examine what they are. As a matter of fact, it was

especially for their sake that we studied the ontogeny of a special

form in such detail; one always sees generalities better if one knows

the specific features of at least one case. What then are the features

of most general and far-reaching importance, which may be abstracted

from the individual history of our sea-urchin, checked always by the

teachings of other ontogenies, including those of plants?

THE FIRST STEPS OF ANALYTICAL MORPHOGENESIS

If we look back upon the long fight of the schools of embryologists in

the eighteenth century about the question whether individual development

was to be regarded as a real production of visible manifoldness or as

a simple growth of visibly pre-existing manifoldness, whether it was

“epigenesis” or “evolutio,” there can be no doubt, if we rely on all

the investigations of the last hundred and fifty years, that, taken in

the descriptive sense, the theory of epigenesis is right. Descriptively

speaking there *is* a production of visible manifoldness in the course

of embryology: that is our first and main result. Any one possessed of

an average microscope may any day convince himself personally that it is

true.

In fact, true epigenesis, in the descriptive sense of the term, does

exist. One thing is formed “after” the other; there is not a mere

“unfolding” of what existed already, though in a smaller form; there is

no “evolutio” in the old meaning of the word.

The word “evolution” in English usually serves to denote the theory of

descent, that is of a real relationship of all organisms. Of course we

are not thinking here of this modern and specifically English meaning of

the Latin word *evolutio*. In its ancient sense it means to a certain

degree just the opposite; it says that there is no formation of anything

new, no transformation, but simply growth, and this is promoted not for

the race but for the individual. Keeping well in mind these historical

differences in the meaning of the word “evolutio,” no mistakes, it seems

to me, can occur from its use. We now shall try to obtain a few more

particular results from our descriptive study of morphogenesis, which

are nevertheless of a general bearing, being real characteristics of

organic individual development, and which, though not calculated of

themselves to further the problem, will in any case serve to prepare for

a more profound study of it.

The totality of the line of morphogenetic facts can easily be resolved

into a great number of distinct processes. We propose to call these

“elementary morphogenetic processes”; the turning in of the endoderm

and its division into three typical parts are examples of them. If we

give the name “elementary organs” to the distinct parts of every stage

of ontogeny which are uniform in themselves and are each the result of

one elementary process in our sense, we are entitled to say that each

embryological stage consists of a certain number of elementary organs.

The mesenchyme ring, the coelum, the middle-intestine, are instances of

such organs. It is important to notice well that the word elementary is

always understood here with regard to visible morphogenesis proper and

does not apply to what may be called elementary in the physiological

sense. An elementary process in our sense is a very distinct act of

form-building, and an elementary organ is the result of every one of

such acts.

The elementary organs are typical with regard to their position and with

regard to their histological properties. In many cases they are of a

very clearly different histological type, as for instance, the cells of

the three so-called germ-layers; and in other cases, though apparently

almost identical histologically, they can be proved to be different

by their different power of resisting injuries or by other means. But

there are not as many different types of histological structure as there

are typically placed organs: on the contrary there are many elementary

organs of the same type in different typical parts of the organism, as

all of you know to be the case with nerves and muscles. It will not be

without importance for our future theory of development, carefully to

notice this fact, that specialisation in the *position* of embryonic

parts is more strict than in their histology.

But elementary organs are not only typical in position and histology,

they are typical also with regard to their form and their relative size.

It agrees with what has been said about histology being independent of

typical position, that there may be a number of organs in an embryonic

stage, all in their most typical positions, which though all possessing

the same histology, may have different forms or different sizes or

both: the single bones of the skeleton of vertebrates or of adult

echinoderms are the very best instances of this most important feature

of organogenesis. If we look back from elementary organs to elementary

processes, the specialisation of the size of those organs may also be

said to be the consequence of a typical duration of the elementary

morphogenetic process leading to them.[7]

[7] The phrase “*ceteris paribus*” has to be added of course, as the

duration of each single elementary morphogenetic process is liable to

vary with the temperature and many other conditions of the medium.

I hardly need to say, that the histology, form, and size of elementary

organs are equally an expression of their present or future

physiological function. At least they prepare for this function by a

specific sort of metabolism which sets in very early.

The whole sequence of individual morphogenesis has been divided by some

embryologists into two different periods; there is a first period,

during which the foundations of the organisation of the “type” are

laid down, and a second period, during which the histo-physiological

specifications are modelled out (von Baer, Götte, Roux). Such a

discrimination is certainly justified, if not taken too strictly; but

its practical application would encounter certain difficulties in many

larval forms, and also, of course, in all plants.

Our mention of plants leads us to the last of our analytical results. If

an animal germ proceeds in its development from a stage *d* to the stage

*g*, passing through *e* and *f*, we may say that the whole of *d* has

become the whole of *f*, but we cannot say that there is a certain part

of *f* which is *d*, we cannot say that *f* is *d* + *a*. But in plants

we can: the stage *f* is indeed equal to *a* + *b* + *c* + *d* + *e* +

*a* [Transcriber’s note: probable typo for *f*] in vegetable organisms;

all earlier stages are actually visible as parts of the last one. The

great embryologist, Carl Ernst von Baer, most clearly appreciated these

analytical differences between animal and vegetable morphogenesis. They

become a little less marked if we remember that plants, in a certain

respect, are not simple individuals but colonies, and that among the

corals, hydroids, bryozoa, and ascidia, we find analogies to plants in

the animal kingdom; but nevertheless the differences we have stated are

not extinguished by such reasoning. It seems almost wholly due to the

occurrence of so many foldings and bendings and migrations of cells and

complexes of cells in animal morphogenesis, that an earlier stage of

their development seems *lost* in the later one; those processes are

almost entirely wanting in plants, even if we study their very first

ontogenetic stages. If we say that almost all production of surfaces

goes on outside in plants, inside in animals, we shall have adequately

described the difference. And this feature again leads to the further

diversity between animals and plants which is best expressed by calling

the former “closed,” the latter “open” forms: animals reach a point

where they are finished, plants never are finished, at least in most

cases.

I hope you will allow that I have tried to draw from descriptive and

comparative embryology as many general analytical results as are

possibly to be obtained. It is not my fault if there are not any

more, nor is it my fault if the results reached are not of the most

satisfactory character. You may say that these results perhaps enable

you to see a little more clearly and markedly than before a few of the

characters of development, but that you have not really learnt anything

new. Your disappointment--my own disappointment--in our analysis is due

to the use of pure description and comparison as scientific methods.

THE LIMITS OF PURE DESCRIPTION IN SCIENCE

We have analysed our descriptions as far as we could, and now we must

confess that what we have found cannot be the last thing knowable

about individual morphogenesis. There must be something deeper to be

discovered: we only have been on the surface of the phenomena, we

now want to get to the very bottom of them. Why then occurs all that

folding, and bending, and histogenesis, and all the other processes we

have described? There must be something that drives them out, so to say.

There is a very famous dictum in the *Treatise on Mechanics* by the

late Gustav Kirchhoff, that it is the task of mechanics to describe

completely and in the most simple manner all the motions that occur in

nature. These words, which may appear problematic even in mechanics,

have had a really pernicious influence on biology. People were extremely

pleased with them. “‘Describing’--that is just what we always have

done,” they said; “now we see that we have done just what was right;

a famous physicist has told us so.” They did not see that Kirchhoff

had added the words “completely and in the most simple manner”; and

moreover, they did not consider that Kirchhoff never regarded it as the

ultimate aim of physics to describe thunderstorms or volcanic eruptions

or denudations; yet it only is with such “descriptions” that biological

descriptions of *given* bodies and processes are to be compared!

Physicists always have used both experiment and hypothetical

construction--Kirchhoff himself did so in the most gifted manner. With

these aids they have gone through the whole of the phenomena, and what

they found to be ultimate and truly elemental, that alone may they

be said to have “described”; but they have “explained” by the aid of

elementalities what proved to be not elemental in itself.[8]

[8] We shall not avoid in these lectures the word “explain”--so much

out of fashion nowadays. To “explain” means to subsume under known

concepts, or rules, or laws, or principles, whether the laws or concepts

themselves be “explained” or not. Explaining, therefore, is always

relative: what is elemental, of course, is only to be described, or

rather to be stated.

It is the *method* of the physicists--not their results--that

morphogenesis has to apply in order to make progress; and this method

we shall begin to apply in our next lectures. Physiology proper has

never been so short-sighted and self-satisfied as not to learn from

other sciences, from which indeed there was very much to be learned; but

morphology has: the bare describing and comparing of descriptions has

been its only aim for about forty years or more, and lines of descent of

a very problematic character were its only general results. It was not

seen that science had to begin, not with problematic events of the past,

but with what actually happens before our eyes.

But before saying any more about the exact rational and experimental

method in morphology, which indeed may be regarded as a new method,

since its prevalence in the eighteenth century had been really

forgotten, we first shall have to analyse shortly some general attempts

to understand morphogenesis by means of hypothetic construction

exclusively. Such attempts have become very important as points of

issue for really exact research, and, moreover, they deserve attention,

because they prove that their authors at least had not quite forgotten

that there were still other problems to be solved in morphology than

only phylogenetical ones.

*B.* EXPERIMENTAL AND THEORETICAL MORPHOGENESIS

1. THE FOUNDATIONS OF THE PHYSIOLOGY OF DEVELOPMENT. “EVOLUTIO” AND

“EPIGENESIS”

THE THEORY OF WEISMANN

Of all the purely hypothetic theories on morphogenesis that of August

Weismann[9] can claim to have had the greatest influence, and to

be at the same time the most logical and the most elaborated. The

“germ-plasma” theory of the German author is generally considered as

being a theory of heredity, and that is true inasmuch as problems of

inheritance proper have been the starting-point of all his hypothetic

speculations, and also form in some respect the most valuable part

of them. But, rightly understood, Weismann’s theory consists of two

independent parts, which relate to morphogenesis and to heredity

separately, and it is only the first which we shall have to take into

consideration at present; what is generally known as the doctrine of the

“continuity of the germ-plasm” will be discussed in a later chapter.

[9] *Das Keimplasma*, Jena, 1892.

Weismann assumes that a very complicated organised structure, below

the limits of visibility even with the highest optical powers, is the

foundation of all morphogenetic processes, in such a way that, whilst

part of this structure is handed over from generation to generation as

the basis of heredity, another part of it is disintegrated during the

individual development, and directs development by being disintegrated.

The expression, “part” of the structure, first calls for some

explanation. Weismann supposes several examples, several copies, as it

were, of his structure to be present in the germ cells, and it is to

these copies that the word “part” has been applied by us: at least one

copy has to be disintegrated during ontogeny.

The morphogenetic structure is assumed to be present in the nucleus

of the germ cells, and Weismann supposes the disintegration of his

hypothetic structure to be accomplished by nuclear division. By the

cleavage of the egg, the most *fundamental* parts of it are separated

one from the other. The word “fundamental” must be understood as

applying not to proper elements or complexes of elements of the

organisation, but to the chief relations of symmetry; the first

cleavage, for instance, may separate the right and the left part of

the structure, the second one its upper and lower parts, and after the

third or equatorial cleavage all the principal eighths of our minute

organisation are divided off: for the minute organisation, it must now

be added, had been supposed to be built up differently in the three

directions of space, just as the adult organism is. Weismann concedes it

to be absolutely unknown in what manner the proper relation between the

parts of the disintegrated fundamental morphogenetic structure and the

real processes of morphogenesis is realised; enough that there may be

imagined such a relation.

At the end of organogenesis the structure is assumed to have been

broken up into its elements, and these elements, which may be chemical

compounds, determine the fate of the single cells of the adult organism.

Here let us pause for a moment. There cannot be any doubt that

Weismann’s theory resembles to a very high degree the old “evolutio”

doctrines of the eighteenth century, except that it is a little less

crude. The chick itself is not supposed to be present in the hen’s egg

before development, and ontogeny is not regarded as a mere growth of

that chick in miniature, but what really is supposed to be present in

the egg is nevertheless a something that in all its parts corresponds

to all the parts of the chick, only under a somewhat different

aspect, while all the relations of the parts of the one correspond

to the relations of the parts of the other. Indeed, only on such an

hypothesis of a fairly fixed and rigid relation between the parts of

the morphogenetic structure could it be possible for the disintegration

of the structure to go on, not by parts of organisation, but by parts

of symmetry; which, indeed, is a very strange, but not an illogical,

feature of Weismann’s doctrine.

Weismann is absolutely convinced that there must be a theory of

“evolutio,” in the old sense of the word, to account for the ontogenetic

facts; that “epigenesis” has its place only in descriptive embryology,

where, indeed, as we know, manifoldness in the *visible* sense is

produced, but that epigenesis can never form the foundation of a real

morphogenetic *theory*: theoretically one pre-existing manifoldness is

transformed into the other. An epigenetic theory would lead right beyond

natural science, Weismann thinks, as in fact, all such theories, if

fully worked out, have carried their authors to vitalistic views. But

vitalism is regarded by him as dethroned for ever.

Under these circumstances we have a good right, it seems to me, to speak

of a *dogmatic* basis of Weismann’s theory of development.

But to complete the outlines of the theory itself: Weismann was

well aware that there were some grave difficulties attaching to his

statements: all the facts of so-called adventitious morphogenesis in

plants, of regeneration in animals, proved that the morphogenetic

organisation could not be fully disintegrated during ontogeny. But these

difficulties were not absolute: they could be overcome: indeed, Weismann

assumes, that in certain specific cases--and he regarded all cases of

restoration of a destroyed organisation as due to specific properties

of the subjects, originated by roundabout variations and natural

selection--that in specific cases, specific arrangements of minute parts

were formed during the process of disintegration, and were surrendered

to specific cells during development, from which regeneration or

adventitious budding could originate if required. “Plasma of reserve”

was the name bestowed on these hypothetic arrangements.

Almost independently another German author, Wilhelm Roux,[10] has

advocated a theoretical view of morphogenesis which very closely

resembles the hypothesis of Weismann. According to Roux a minute

ultimate structure is present in the nucleus of the germ and directs

development by being divided into its parts during the series of nuclear

divisions.

[10] *Die Bedeutung der Kernteilungsfiguren*, Leipzig, 1883.

But in spite of this similarity of the outset, we enter an altogether

different field of biological investigation on mentioning Roux’s name:

we are leaving hypothetic construction, at least in its absoluteness,

and are entering the realms of scientific experiment in morphology.

EXPERIMENTAL MORPHOLOGY

I have told you already in the last lecture that, while in the

eighteenth century individual morphogenesis had formed the centre of

biological interest and been studied experimentally in a thoroughly

adequate manner, that interest gradually diminished, until at last

the physiology of form as an exact separate science was almost wholly

forgotten. At least that was the state of affairs as regards zoological

biology; botanists, it must be granted, have never lost the historical

continuity to such a degree; botany has never ceased to be regarded

as one science and never was broken up into parts as zoology was.

Zoological physiology and zoological morphology indeed were for many

years in a relationship to one another not very much closer than the

relation between philology and chemistry.

There were always a few men, of course, who strove against the current.

The late Wilhelm His,[11] instance, described the embryology of the

chick in an original manner, in order to find out the mechanical

relations of embryonic parts, by which passive deformation, as an

integrating part of morphogenesis, might be induced. He also most

clearly stated the ultimate aim of embryology to be the mathematical

derivation of the adult form from the distribution of growth in the

germ. To Alexander Goette[12] we owe another set of analytical

considerations about ontogeny. Newport, as early as 1850, and in

later years Pflüger and Rauber, carried out experiments on the eggs

of the frog, which may truly be called anticipatory of what was to

follow. But it was Wilhelm Roux,[13] now professor of anatomy at

Halle, who entered the field with a thoroughly elaborated programme,

who knew not only how to state the problem analytically, but also

how to attack it, fully convinced of the importance of what he did.

“Entwickelungsmechanik,”--mechanics of development--he called the “new

branch of anatomical science” of which he tried to lay the foundations.

[11] *Unsere Körperform*, Leipzig, 1875.

[12] *Die Entwickelungsgeschichte der Unke*, Leipzig, 1875.

[13] *Gesammelte Abhandlungen*, Leipzig, 1895. Most important

theoretical papers:--*Zeitschr. Biolog.* 21, 1885; *Die

Entwickelungsmechanik der Organismen*, Wien, 1890; *Vorträge und

Aufsätze über Entwickelungsmechanik*, Heft i., Leipzig, 1905.

I cannot let this occasion pass without emphasising in the most decided

manner how highly in my opinion Roux’s services to the systematic

exploration of morphogenesis must be esteemed. I feel the more obliged

to do so, because later on I shall have to contradict not only many

of his positive statements but also most of his theoretical views. He

himself has lately given up much of what he most strongly advocated only

ten years ago. But Roux’s place in the history of biological science can

never be altered, let science take what path it will.

It is not the place here to develop the logic of experiment; least of

all is it necessary in the country of John Stuart Mill. All of you know

that experiment, by its method of isolating the single constituents

of complicated phenomena, is the principal aid in the discovery of

so-called causal relations. Let us try then to see what causal

relations Wilhelm Roux established with the aid of morphogenetic

experiment.

THE WORK OF WILHELM ROUX

We know already that an hypothesis about the foundation of individual

development was his starting-point. Like Weismann he supposed that

there exists a very complicated structure in the germ, and that nuclear

division leads to the disintegration of that structure. He next tried to

bring forward what might be called a number of indicia supporting his

view.

A close relation had been found to exist in many cases between the

direction of the first cleavage furrows of the germ and the direction

of the chief planes of symmetry in the adult: the first cleavage, for

instance, very often corresponds to the median plane, or stands at right

angles to it. And in other instances, such as have been worked out into

the doctrine of so-called “cell-lineages,” typical cleavage cells were

found to correspond to typical organs. Was not that a strong support

for a theory which regarded cellular division as the principal means

of differentiation? It is true, the close relations between cleavage

and symmetry did not exist in every case, but then there had always

happened some specific experimental disturbances, *e.g.* influences of

an abnormal direction of gravity on account of a turning over of the

egg, and it was easy to reconcile such cases with the generally accepted

theory on the assumption of what was called “anachronism” of cleavage.

But Roux was not satisfied with mere indicia, he wanted a proof, and

with this intention he carried out an experiment which has become

very celebrated.[14] With a hot needle he killed one of the first two

blastomeres of the frog’s egg after the full accomplishment of its first

cleavage, and then watched the development of the surviving cell. A

typical half-embryo was seen to emerge--an organism indeed, which was as

much a half as if a fully formed embryo of a certain stage had been cut

in two by a razor. It was especially in the anterior part of the embryo

that its “halfness” could most clearly be demonstrated.

[14] *Virchow’s Archiv.* 114, 1888.

That seemed to be a proof of Weismann’s and Roux’s theory of

development, a proof of the hypothesis that there is a very complicated

structure which promotes ontogeny by its disintegration, carried out

during the cell divisions of embryology by the aid of the process of

nuclear division, the so-called “karyokinesis.”

To the dispassionate observer it will appear, I suppose, that the

conclusions drawn by Roux from his experiment go a little beyond their

legitimate length. Certainly some sort of “evolutio” is proved by

rearing half the frog from half the egg. But is anything proved, is

there anything discovered at all about the nucleus? It was only on

account of the common opinion about the part it played in morphogenesis

that the nucleus had been taken into consideration.

Things soon became still more ambiguous.

THE EXPERIMENTS ON THE EGG OF THE SEA-URCHIN

Roux’s results were published for the first time in 1888; three years

later I tried to repeat his fundamental experiment on another subject

and by a somewhat different method. It was known from the cytological

researches of the brothers Hertwig and Boveri that the eggs of the

common sea-urchin (*Echinus microtuberculatus*) are able to stand well

all sorts of rough treatment, and that, in particular, when broken into

pieces by shaking, their fragments will survive and continue to segment.

I took advantage of these facts for my purposes. I shook the germs

rather violently during their two-cell stage, and in several instances I

succeeded in killing one of the blastomeres, while the other one was not

damaged, or in separating the two blastomeres from one another.[15]

[15] *Zeitschr. wiss. Zool.* 53, 1891.

Let us now follow the development of the isolated surviving cell. It

went through cleavage just as it would have done in contact with its

sister-cell, and there occurred cleavage stages which were just half

of the normal ones. The stage, for instance, which corresponded to the

normal sixteen-cell stage, and which, of course, in my subjects was

built up of eight elements only, showed two micromeres, two macromeres

and four cells of medium size, exactly as if a normal sixteen-cell stage

had been cut in two; and the form of the whole was that of a hemisphere.

So far there was no divergence from Roux’s results.

The development of our Echinus proceeds rather rapidly, the cleavage

being accomplished in about fifteen hours. I now noticed on the evening

of the first day of the experiment, when the half-germ was composed of

about two hundred elements, that the margin of the hemispherical germ

bent together a little, as if it were about to form a whole sphere

of smaller size, and, indeed, the next morning a *whole* diminutive

blastula was swimming about. I was so much convinced that I should get

Roux’s morphogenetical result in all its features that, even in spite of

this whole blastula, I now expected that the next morning would reveal

to me the half-organisation of my subject once more; the intestine, I

supposed, might come out quite on one side of it, as a half-tube, and

the mesenchyme ring might be a half one also.

But things turned out as they were bound to do and not as I had

expected; there was a typically *whole* gastrula on my dish the next

morning, differing only by its small size from a normal one; and this

*small but whole* gastrula was followed by a whole and typical small

pluteus-larva (Fig. 5).

[Illustration: Fig. 5.--Illustration of Experiments on Echinus.

*a*_1 and *b*_1. Normal gastrula and normal pluteus.

*a*_2 and *b*_2. “Half”-gastrula and “half”-pluteus, that *ought* to result

from one of the first two blastomeres, when isolated, according to the

theory of “evolutio.”

*a*_3 and *b*_3. The small *but whole* gastrula and pluteus that actually

*do* result.]

That was just the opposite of Roux’s result: one of the first two

blastomeres had undergone a half-cleavage as in his case, but then it

had become a whole organism by a simple process of rearrangement of its

material, without anything that resembled regeneration, in the sense of

a completion by budding from a wound.

If one blastomere of the two-cell stage was thus capable of performing

the morphogenetical process in its totality, it became, of course,

*impossible* to allow that nuclear division had separated any sort of

“germ-plasm” into two different halves, and not even the protoplasm of

the egg could be said to have been divided by the first cleavage furrow

into unequal parts, as the postulate of the strict theory of so-called

“evolutio” had been. This was a very important result, sufficient

alone to overthrow at once the theory of ontogenetical “evolutio,”

the “Mosaiktheorie” as it had been called--not by Roux himself, but

according to his views--in its exclusiveness.

After first widening the circle of my observations by showing that

one of the first four blastomeres is capable of performing a whole

organogenesis, and that three of the first four blastomeres together

result in an absolutely perfect organism, I went on to follow up

separately one of the two fundamental problems which had been suggested

by my first experiment: was there anything more to find out about

the importance or unimportance of the single *nuclear* divisions in

morphogenesis?[16]

[16] *Zeitschr. wiss. Zool.* 55, 1892.

By raising the temperature of the medium or by diluting the sea-water to

a certain degree it proved at first to be possible to alter in a rather

fundamental way the type of the cleavage-stages without any damage to

the resulting organism. There may be no micromeres at the sixteen-cell

stage, or they may appear as early as in the stage of eight cells;

no matter, the larva is bound to be typical. So it certainly is not

necessary for all the cleavages to occur just in their normal order.

But of greater importance for our purposes was what followed. I

succeeded in pressing the eggs of Echinus between two glass plates,

rather tightly, but without killing them; the eggs became deformed to

comparatively flat plates of a large diameter. Now in these eggs all

nuclear division occurred at right angles to the direction of pressure,

that is to say, in the direction of the plates, as long as the pressure

lasted; but the divisions began to occur at right angles to their former

direction, as soon as the pressure ceased. By letting the pressure be

at work for different times I therefore, of course, had it quite in my

power to obtain cleavage types just as I wanted to get them. If, for

instance, I kept the eggs under pressure until the eight-cell stage was

complete, I got a plate of eight cells one beside the other, instead of

two rings, of four cells each, one above the other, as in the normal

case; but the next cell division occurred at right angles to the former

ones, and a sixteen-cell stage, of two plates of eight cells each, one

above the other, was the result. If the pressure continued until the

sixteen-cell stage was reached, sixteen cells lay together in one plate,

and two plates of sixteen cells each, one above the other, were the

result of the next cleavage.

We are not, however, studying these things for cytological, but for

morphogenetical purposes, and for these the cleavage phenomenon itself

is less important than the organogenetic result of it: all our subjects

resulted in *absolutely normal* organisms. Now, it is clear, that the

spatial relations of the different nuclear divisions to each other are

anything but normal, in the eggs subjected to the pressure experiments;

that, so to say, every nucleus has got quite different neighbours if

compared with the “normal” case. If that makes no difference, then

there *cannot* exist any close relation between the single nuclear

divisions and organogenesis at all, and the conclusion we have drawn

more provisionally from the whole development of isolated blastomeres

has been extended and proved in the most perfect manner. There ought to

result a morphogenetic chaos according to the theory of real “evolutio”

carried out by nuclear division, if the positions of the single nuclei

were fundamentally changed with regard to one another (Fig. 6). But now

there resulted not chaos, but the normal organisation: therefore it was

disproved in the strictest way that nuclear divisions have any bearing

on the origin of organisation; at least as far as the divisions during

cleavage come into account.

[Illustration: Fig. 6.--Pressure-experiments on Echinus.

*a*_1 and *b*_1. Two normal cleavage stages, consisting of eight and

sixteen cells.

*a*_2 and *b*_2. Corresponding stages modified by exerting pressure

until the eight-cell stage was finished. See text.]

On the egg of the frog (O. Hertwig), and on the egg of annelids (E. B.

Wilson), my pressure experiments have been carried out with the same

result.[17]

[17] In the pressure experiments I had altered the relative position of

the nuclei *in origine*. In later years I succeeded in disturbing the

arrangement of the fully formed cells of the eight-cell stage, and in

getting normal larvæ in spite of that in many cases. But as this series

of experiments is not free from certain complications--which in part

will be understood later on (see page 73)--it must suffice here to have

mentioned them. (For further information see my paper in *Archiv. f.

Entwickelungsmechanik*, xiv., 1902, page 500.)

ON THE INTIMATE STRUCTURE OF THE PROTOPLASM OF THE GERM

Nuclear division, as we have seen, cannot be the basis of organogenesis,

and all we know about the whole development of isolated blastomeres

seems to show that there exists nothing responsible for differentiation

in the protoplasm either.

But would that be possible? It cannot appear possible on a more profound

consideration of the nature of morphogenesis, it seems to me: as the

untypical agents of the medium cannot be responsible in any way for

the origin of a form combination which is most typical and specific,

there must be somewhere in the egg itself a certain factor which is

responsible at least for the general orientation and symmetry of it.

Considerations of this kind led me, as early as 1893,[18] to urge the

hypothesis that there existed, that there *must* exist, a sort of

intimate structure in the egg, including polarity and bilaterality as

the chief features of its symmetry, a structure which belongs to every

smallest element of the egg, and which might be imagined by analogy

under the form of elementary magnets.[19] This hypothetic structure

could have its seat in the protoplasm only. In the egg of echinoderms it

would be capable of such a quick rearrangement after being disturbed,

that it could not be observed but only inferred logically; there might,

however, be cases in which its real discovery would be possible. Indeed

Roux’s frog-experiment seems to be a case where it is found to be at

work: at least it seems very probable to assume that Roux obtained half

of a frog’s embryo because the protoplasm of the isolated blastomere had

preserved the “halfness” of its intimate structure, and had not been

able to form a small whole out of it.

[18] *Mitteil. Neapel. 11, 1893.*

[19] But the elementary magnets would have to be bilateral!

Of course it was my principal object to verify this hypothesis, and

such verification became possible in a set of experiments which my

friend T. H. Morgan and myself carried out together,[20] in 1895, on

the eggs of ctenophores, a sort of pelagic animals, somewhat resembling

the jelly-fish, but of a rather different inner organisation. The

zoologist Chun had found even before Roux’s analytical studies, that

isolated blastomeres of the ctenophore egg behave like parts of the

whole and result in a half-organisation like the frog’s germ does. Chun

had not laid much stress on his discovery, which now, of course, from

the new points of view, became a very important one. We first repeated

Chun’s experiment and obtained his results, with the sole exception

that there was a tendency of the endoderm of the half-larva of Beroë

to become more than “half.” But that was not what we chiefly wanted to

study. We succeeded in cutting away a certain mass of the protoplasm

of the ctenophore egg just before it began to cleave, without damaging

its nuclear material in any way: in all cases, where the cut was

performed at the side, there resulted a certain type of larvae from

our experiments which showed exactly the same sort of defects as were

present in larvae developed from one of the first two blastomeres alone.

[20] *Arch. Entw. Mech.* 2, 1895.

The hypothesis of the morphogenetic importance of *protoplasm* had thus

been proved. In our experiments there was all of the nuclear material,

but there were defects on one side of the protoplasm of the egg; and the

defects in the adult were found to correspond to these defects in the

protoplasm.

And now O. Schultze and Morgan succeeded in performing some experiments

which directly proved the hypothesis of the part played by protoplasm

in the subject employed by Roux, *viz.*, the frog’s egg. The first of

these investigators managed to rear two whole frog embryos of small

size, if he slightly pressed the two-cell stage of that form between

two plates of glass and turned it over; and Morgan,[21] after having

killed one of the first two blastomeres, as was done in the original

experiment of Roux, was able to bring the surviving one to a half or

to a whole development according as it was undisturbed or turned.

There cannot be any doubt that in both of these cases, it is the

possibility of a rearrangement of protoplasm, offered by the turning

over, which allows the isolated blastomere to develop as a whole. The

regulation of the frog’s egg, with regard to its becoming whole, may be

called facultative, whilst the same regulation of the egg of Echinus

is obligatory. It is not without interest to note that the first two

blastomeres of the common newt, *i.e.* of a form which belongs to the

other class of Amphibia, after a separation of *any* kind, *always*

develop as wholes, their faculty of regulation being obligatory, like

that of Echinus.

[21] *Anat. Anz.* 10, 1895.

Whole or partial development may thus be dependent on the power of

regulation contained in the intimate polar-bilateral structure of the

protoplasm. Where this is so, the regulation and the differences in

development are both connected with the chief relations of symmetry.

The development becomes a half or a quarter of the normal because

there is only one-half or one-quarter of a certain structure present,

one-half or one-quarter with regard to the very wholeness of this

structure; the development is whole, in spite of disturbances, if

the intimate structure became whole first. We may describe the

“wholeness,” “halfness,” or “quarterness” of our hypothetic structure

in a mathematical way, by using three axes, at right angles to one

another, as the base of orientation. To each of these, *x*, *y*, and

*z*, a certain specific state with regard to the symmetrical relations

corresponds; thence it follows that, if there are wanting all those

parts of the intimate structure which are determined, say, by a negative

value of *y*, by minus *y*, then there is wanting half of the intimate

structure; and this halfness of the intimate structure is followed by

the halfness of organogenesis, the dependence of the latter on the

intimate structure being established. But if regulation has restored,

on a smaller scale, the whole of the arrangement according to all values

of *x*, *y* and *z*, development also can take place completely (Fig. 7).

[Illustration: Fig. 7.--Diagram illustrating the intimate Regulation of

Protoplasm from “Half” to “Whole.”

The large circle represents the original structure of the egg. In all

cases where cleavage-cells of the two-cell stage are isolated this

original structure is only present as “half” in the beginning, say

only on the right (+*y*) side. Development then becomes “half,” if the

intimate structure remains half; but it becomes “whole” (on a smaller

scale) if a new whole-structure (small circle!) is formed by regulatory

processes.]

I am quite aware that such a discussion is rather empty and purely

formal, nevertheless it is by no means without value, for it shows

most clearly the differences between what we have called the intimate

structure of germs, responsible only for the general symmetry of

themselves and of their isolated parts, and another sort of possible

structure of the egg-protoplasm which we now shall have to consider, and

which, at the first glance, seems to form a serious difficulty to our

statements, as far at least as they claim to be of general importance.

The study of this other sort of germinal structure at the same time will

lead us a step farther in our historical sketch of the first years of

“Entwickelungsmechanik” and will bring this sketch to its end.

ON SOME SPECIFICITIES OF ORGANISATION IN CERTAIN GERMS

It was known already about 1890, from the careful study of what has

been called “cell-lineage,” that in the eggs of several families of

the animal kingdom the origin of certain organs may be traced back to

individual cells of cleavage, having a typical histological character

of their own. In America especially such researches have been carried

out with the utmost minuteness, E. B. Wilson’s study of the cell-lineage

of the Annelid *Nereis* being the first of them. If it were true that

nuclear division is of no determining influence upon the ontogenetic

fate of the blastomeres, only peculiarities of the different parts of

the protoplasm could account for such relations of special cleavage

cells to special organs. I advocated this view as early as in 1894,

and it was proved two years later by Crampton, a pupil of Wilson’s,

in some very fine experiments performed on the germ of a certain

mollusc.[22] The egg of this form contains a special sort of protoplasm

near its vegetative pole, and this part of it is separated at each

of the first two segmentations by a sort of pseudo-cleavage, leading

to stages of three and five separated masses instead of two and four,

the supernumerary mass being the so-called “yolk-sac” and possessing

no nuclear elements (Fig. 8). Crampton removed this yolk-sac at the

two-cell stage, and he found that the cleavage of the germs thus

operated upon was normal except with regard to the size and histological

appearance of one cell, and that the larvae originating from these

germs were complete in every respect except in their mesenchyme, which

was wanting. A special part of the protoplasm of the egg had thus been

brought into relation with quite a special part of organisation, *and

that special part of the protoplasm contained no nucleus*.

[22] *Arch. Entw. Mech.* 3, 1896.

[Illustration: Fig. 8.--The Mollusc Dentalium (*after* E. B. Wilson).

*a.* The egg, consisting of three different kinds of protoplasmatic

material.

*b.* First cleavage-stage. There are two cells and one “pseudo-cell,”

the yolk-sac, which contains no nucleus. This was removed in Crampton’s

experiment.]

GENERAL RESULTS OF THE FIRST PERIOD OF “ENTWICKELUNGSMECHANIK”

This experiment of Crampton’s, afterwards confirmed by Wilson himself,

may be said to have closed the first period of the new science of

physiology of form, a period devoted almost exclusively to the problem

whether the theory of nuclear division or, in a wider sense, whether the

theory of a strict “evolutio” as the basis of organogenesis was true or

not.

It was shown, as we have seen, that the theory of the “qualitatively

unequal nuclear division” (“qualitativ-ungleiche Kernteilung” in German)

certainly was not true, and that there also was no strict “evolutio”

in protoplasm. Hence Weismann’s theory was clearly disproved. There

certainly is a good deal of real “epigenesis” in ontogeny, a good deal

of “production of manifoldness,” not only with regard to visibility but

in a more profound meaning. But some sort of pre-formation had also

been proved to exist, and this pre-formation, or, if you like, this

restricted evolution, was found to be of two different kinds. First an

intimate organisation of the protoplasm, spoken of as its polarity and

bilaterality, was discovered, and this had to be postulated for every

kind of germs, even when it was overshadowed by immediate obligatory

regulation after disturbances. Besides that there were cases in which

a real specificity of special parts of the germ existed, a relation of

these special parts to special organs: but this sort of specification

also was shown to belong to the protoplasm.

It follows from all we have mentioned about the organisation of

protoplasm and its bearing on morphogenesis, that the eggs of different

animals may behave rather differently, in this respect, and that

the eggs indeed may be classified according to the degree of their

organisation. Though we must leave a detailed discussion of these

topics to morphology proper, we yet shall try shortly to summarise

what has been ascertained about them in the different classes of the

animal kingdom. A full regulation of the *intimate* structure of

isolated blastomeres to a new whole, has been proved to exist in the

highest degree in the eggs of all echinoderms, medusae, nemertines,

Amphioxus, fishes, and in one class of the Amphibia (the *Urodela*);

it is facultative only among the other class of Amphibia, the *Anura*,

and seems to be only partly developed or to be wanting altogether among

ctenophora, ascidia, annelids, and mollusca. Peculiarities in the

organisation of *specific parts* of protoplasm have been proved to occur

in more cases than at first had been assumed; they exist even in the

echinoderm egg, as experiments of the last few years have shown; even

here a sort of specification exists at the vegetative pole of the egg,

though it is liable to a certain kind of regulation; the same is true in

medusae, nemertines, etc.; but among molluscs, ascidians, and annelids

no regulation about the specific organisation of the germ in cleavage

has been found in any case.

The differences in the degree of regulability of the intimate germinal

structure may easily be reduced to simple differences in the physical

consistency of their protoplasm.[23] But all differences in specific

organisation must remain as they are for the present; it will be one of

the aims of the future theory of development to trace these differences

also to a common source.

[23] It deserves notice in this connection, that in some cases the

protoplasm of parts of a germ has been found to be more regulable in

the earliest stages, when it is very fluid, than later, when it is more

stiff.

That such an endeavour will probably be not without success, is clear,

I should think, from the mere fact that differences with regard to

germinal specific pre-formation do not agree in any way with the

systematic position of the animals exhibiting them; for, strange as it

would be if there were two utterly different kinds of morphogenesis, it

would be still more strange if there were differences in morphogenesis

which were totally unconnected with systematic relationship: the

ctenophores behaving differently from the medusae, and Amphioxus

differently from ascidians.

SOME NEW RESULTS CONCERNING RESTITUTIONS

We now might close this chapter, which has chiefly dealt with the

disproof of a certain sort of ontogenetic theories, and therefore

has been almost negative in its character, did it not seem desirable

to add at least a few words about the later discoveries relating to

morphogenetic restorations of the adult. We have learnt that Weismann

created his concept of “reserve plasma” to account for what little

he knew about “restitutions”: that is, about the restoration of lost

parts: he only knew regeneration proper in animals and the formation of

adventitious buds in plants. It is common to both of these phenomena

that they take their origin from typically localised points of the body

in every case; each time they occur a certain well-defined part of the

body is charged with the restoration of the lost parts. To explain

such cases Weismann’s hypothesis was quite adequate, at least in a

logical sense. But at present, as we shall discuss more fully in another

chapter, we know of some very widespread forms of restitution, in which

what is to be done for a replacement of the lost is not entrusted to

*one* typical part of the body in every case, but in which the whole

of the morphogenetic action to be performed is transferred in its

*single* parts to the *single* parts of the body which is accomplishing

restoration: each of its parts has to take an individual share in the

process of restoration, effecting what is properly called a certain kind

of “re-differentiation” (“Umdifferenzierung”), and this share varies

according to the relative position of the part in each case. Later on

these statements will appear in more correct form than at present, and

then it will become clear that we are fully entitled to emphasise at the

end of our criticism of Weismann’s theory, that his hypothesis relating

to restorations can be no more true than his theory of development

proper was found to be.

And now we shall pass on to our positive work.

We shall try to sketch the outlines of what might properly be called an

*analytical theory of morphogenesis*; that is, to explain the sum of our

knowledge about organic form-production, gained by experiment and by

logical analysis, in the form of a real system, in which each part will

be, or at least will try to be, in its proper place and in relation with

every other part. Our analytical work will give us ample opportunity of

mentioning many important topics of so-called general physiology also,

irrespective of morphogenesis as such. But morphogenesis is always to

be the centre and starting-point of our analysis. As I myself approach

the subject as a zoologist, animal morphogenesis, as before, will be the

principal subject of what is to follow.

2. ANALYTICAL THEORY OF MORPHOGENESIS[24]

[24] Compare my *Analytische Theorie der organischen Entwickelung*,

Leipzig, 1894, and my reviews in *Ergebnisse der Anatomie und

Entwickelungsgeschichte*, vols. viii. xi. xiv., 1899-1905. A shorter

review is given in *Ergebnisse der Physiologie*, vol. v., 1906. The full

literature will be found in these reviews.

α. THE DISTRIBUTION OF MORPHOGENETIC POTENCIES

*Prospective Value and Prospective Potency*

Wilhelm Roux did not fail to see that the questions of the locality and

the time of all morphogenetic differentiations had to be solved first,

before any problem of causality proper could be attacked. From this

point of view he carried out his fundamental experiments.

It is only in terminology that we differ from his views, if we prefer

to call our introductory chapter an analysis of the distribution of

morphogenetic potencies. The result will be of course rather different

from what Roux expected it would be.

Let us begin by laying down two fundamental concepts. Suppose we have

here a definite embryo in a definite state of development, say a

blastula, or a gastrula, or some sort of larva, then we are entitled

to study any special element of any special elementary organ of this

germ with respect to what is actually to develop out of this very

element in the future actual course of this development, whether it be

undisturbed or disturbed in any way; it is, so to say, the actual, *the

real fate* of our element, that we take in account. I have proposed to

call this real fate of each embryonic part in this very definite line

of morphogenesis its *prospective value* (“prospective Bedeutung” in

German). The fundamental question of the first chapter of our analytical

theory of development may now be stated as follows: Is the prospective

value of each part of any state of the morphogenetic line constant,

*i.e.* is it unchangeable, can it be nothing but one; or is it variable,

may it change according to different circumstances?

We first introduce a second concept: the term *prospective potency*

(“prospective Potenz” in German) of each embryonic element. The term

“prospective morphogenetic potency” is to signify the *possible

fate* of each of those elements. With the aid of our two artificial

concepts we are now able to formulate our introductory question thus:

Is the prospective potency of each embryonic part fully given by

its prospective value in a certain definite case; is it, so to say,

identical with it, or does the prospective potency contain more than the

prospective value of an element in a certain case reveals?

We know already from our historical sketch that the latter is true: that

the actual fate of a part need not be identical with its possible fate,

at least in many cases; that the potency of the first four blastomeres

of the egg of the sea-urchin, for instance, has a far wider range than

is shown by what each of them actually performs in even this ontogeny.

There are more morphogenetic possibilities contained in each embryonic

part than are actually realised in a special morphogenetic case.

As the most important special morphogenetic case is, of course, the

so-called “normal” one, we can also express our formula in terms of

special reference to it: there are more morphogenetic possibilities in

each part than the observation of the normal development can reveal.

Thus we have at once justified the application of analytical experiment

to morphogenesis, and have stated its most important results.

As the introductory experiments about “Entwickelungsmechanik” have shown

already that the prospective potency of embryonic parts, at least in

certain cases, *can* exceed their prospective value--that, at least in

certain cases, it can be different from it--the concept of prospective

potency at the very beginning of our studies puts itself in the centre

of analytical interest, leaving to the concept of prospective value the

second place only. For that each embryonic part actually has a certain

prospective value, a specified actual fate in every single case of

ontogeny, is clear from itself and does not affirm more than the reality

of morphogenetic cases in general; but that the prospective value of the

elements may change, that there is a morphogenetic power in them, which

contains more than actuality; in other words, that the term “prospective

potency” has not only a logical but a factual interest: all these points

amount to a statement not only of the most fundamental introductory

results but also of the actual *problems* of the physiology of form.

If at each point of the germ something else *can* be formed than

actually is formed, why then does there happen in each case just what

happens and nothing else? In these words indeed we may state the chief

problem of our science, at least after the fundamental relation of

the superiority of prospective potency to prospective value has been

generally shown.

We consequently may shortly formulate our first problem as the question

of the distribution of the prospective morphogenetic potencies in the

germ. Now this general question involves a number of particular ones.

Up to what stage, if at all, is there an absolutely equal distribution

of the potencies over all the elements of the germ? When such an equal

distribution has ceased to exist at a certain stage, what are then the

relations between the parts of different potency? How, on the other

hand, does a newly arisen, more specialised sort of potency behave with

regard to the original general potency, and what about the distribution

of the more restricted potency?

I know very well that all such questions will seem to you a little

formal, and, so to say, academical at the outset. We shall not fail to

attach to them very concrete meanings.

*The Potencies of the Blastomeres*

At first we turn back to our experiments on the egg of the sea-urchin

as a type of the germ in the very earliest stages. We know already that

each of the first two, or each of the first four, or three of the first

four blastomeres together may produce a whole organism. We may add that

the swimming blastula, consisting of about one thousand cells, when cut

in two quite at random, in a plane coincident with, or at least passing

near, its polar axis, may form two fully developed organisms out of its

halves.[25] We may formulate this result in the words: the prospective

potency of the single cells of a blastula of Echinus is the same for

all of them; their prospective value is as far as possible from being

constant.

[25] If the plane of section passes near the equator of the germ, two

whole larvae may be formed also, but in the majority of cases the

“animal” half does not go beyond the blastula. The specific features of

the organisation of the protoplasm come into account here. See also page

65, note 1.

But we may say even a little more: what actually will happen in each of

the blastula cells in any special case of development experimentally

determined depends on the position of that cell in the whole, if the

“whole” is put into relation with any fixed system of co-ordinates; or

more shortly, “the prospective value of any blastula cell is a function

of its position in the whole.”

I know from former experience that this statement wants a few words of

explanation. The word “function” is employed here in the most general,

mathematical sense, simply to express that the prospective value,

the actual fate of a cell, will change, whenever its position in the

whole is different.[26] The “whole” may be related to any three axes

drawn through the normal undisturbed egg, on the hypothesis that there

exists a primary polarity and bilaterality of the germ; the axes which

determine this sort of symmetry may, of course, conveniently be taken as

co-ordinates; but that is not necessary.

[26] A change of the position of the cell is of course effected by each

variation of the direction of the cut, which is purely a matter of

chance.

*The Potencies of Elementary Organs in General*