THE CELL-THEORY.
With the founding of the cell-theory by Schwann in 1839 an important
step was taken in the analysis of the degrees of composition of the
animal body. Aristotle had distinguished three--the unorganised
material, itself compounded of the four primitive elements, earth and
water, air and fire, the homogeneous parts or tissues and the
heterogeneous parts or organs, and this conception was retained with
little change even to the days of Cuvier and von Baer. Those of the old
anatomists who speculated on the relations of organic elements to one
another were dominated by Aristotle's simple and profound
classification, and proposed schemes which differed from his only in
detail. Bichat enlarged and deepened the concept of tissue, but the
degree of composition below this was for him, as for all anatomists of
his time, a fibrous or pulpy "cellulosity," living, indeed, but showing
no uniform and elemental structure. It was Schwann's merit to interpose
between the tissue and the mere unorganised material a new element of
structure, the cell. And, as it happened, a few years before Schwann
published his cell-theory, Dujardin hinted at another degree of
composition which was later to take its place between the cell and the
chemical elements--sarcode or protoplasm.
As is well known, the concept of the cell arose first in botany. Robert
Hooke discovered cells in cork and pith in 1667, and his discovery was
followed up by Grew and Malpighi in 1671, and by Leeuenhoek in 1695. But
they did not conceive the cell as a living, independent, structural
unit. They were interested in the physiology of the plant as a whole,
how it lived and nourished itself, and they studied cells and
sieve-tubes, wood fibres and tracheae with a view rather to finding out
their functions and their significance for the life of the plant than to
discovering the minutiae of their structure. The same attitude was taken
up by the few botanists who in the 18th century paid any heed to the
microscopical anatomy of plants. For C. F. Wolff,[238] the formation of
cells was a result of the secretion of drops of sap in the fundamental
substance of the plant, this substance remaining as cell-walls when
cell-formation was completed--no idea here of cells as units of
structure.
In the early 19th century, interest in plant anatomy revived somewhat,
and much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von
Mohl.[239] As a result of their work the fact was established that the
tissues of plants are composed of elements which can, with few
exceptions, be reduced to one simple fundamental form--the spherical
closed cell. Thus the vessels of plants are formed by coalescence of
cells, fibres by the elongation of cells and the thickening and
toughening of their walls. At this time, interest was concentrated on
the cell-wall, to the almost total neglect of the cell-contents; the
"matured framework" of plant cells, to use Sach's convenient phrase, was
the chief, almost the sole, object of study. And it was natural enough
that the mere architecture of the plant should monopolise interest, that
the composition of the tissues out of the cells, and the fitting
together of the tissues to form the plant should awaken and hold the
curiosity of the investigator; even the modifications of the cell-walls
themselves, their rings and spiral thickenings and pits, offered a
fascinating field of enquiry.
The idea that the cell-contents might show a characteristic and
individual structure had hardly dawned upon botanists when Schleiden
published his famous paper, _Beitraege zur Phytogenesis_.[240] Schleiden's
theme in this paper is the origin and development of the plant cell, a
subject then very obscure, in spite of pioneer work by Mirbel. A few
years before, Robert Brown had called attention to the presence in the
epidermal cells of orchids and other plants of a characteristic spot
which he called the areola or nucleus.[241] Schleiden saw the importance
of this discovery, confirmed the constant presence of the nucleus in
young cells, and held it to be an elementary organ of the cell. He named
it the cytoblast because, in his opinion, it formed the cell. It was
embedded in a peculiar gummy substance, the cytoblastem, which formed a
lining to the cellulose cell-wall. Within the nucleus there was often a
small dark spot or sphere--the nucleolus. The nucleus, Schleiden
thought, originated as a minute granule in the cytoblastem which
gradually increased in size, becoming first a nucleolus (_Kernchen_),
and then, by further condensation of matter round it, a nucleus. Several
nuclei might be formed in this way in a single cell. New cells took
their origin directly from a full-grown nucleus, in a peculiar way which
Schleiden describes as follows:--"As soon as the cytoblasts have reached
their full size a delicate transparent vesicle arises on their surface;
this is the young cell, which at first takes the shape of a very flat
segment of a sphere, of which the plane surface is formed by the
cytoblast, the convex side by the young cell itself, which lies upon the
cytoblast like a watch-glass on a watch" (p. 145). The young cells
increase in size and fill up the cavity of the old cell, which is in
time resorbed. Cell-development always takes place within existing
cells, and either one or many new cells may be formed within the
mother-cell. Schleiden's views on cell-formation were drawn from some
rather imperfect observations on the embryo-sac and pollen-tube, but he
extended his theory to cell-formation in general. Though wrong in almost
all respects the theory had at least the merit of fixing attention upon
the really important constituents of the cell, the nucleus and the
cell-plasma. To Schleiden, too, we owe the conception of the cell as a
more or less independent living unity, whose life is not entirely
identified with the life of the plant as a whole. "Each cell," he
writes, "carries on a double life; one a quite independent and
self-contained life, the other a dependent life in so far as the cell
has become an integral part of the plant" (p. 138).
So long as the definition of the plant cell embraced little more than
the hardened cell-wall it was little wonder that "cells" in this sense
were not recognised in animal tissues, except in a few exceptional
cases--as in the notochord by Johannes Mueller.[242] Careful observation of
animal tissues discovered in some cases the existence of discontinuous
units of structure, but these were not, as a rule, recognised before
1838 as analogous to plant cells. Von Baer, for example, observed that
the young chick embryo was composed partly of an albuminous mass and
partly of _Kuegelchen_ or little globules suspended in it
(_Entwickelungsgeschichte_, i., pp. 19, 144). Since such _Kuegelchen_
disposed in a row formed the notochord (i., p. 145) it seems probable
that his _Kuegelchen_ were really cells. Similarly A. de Quatrefages[243]
in 1834 saw and figured segmentation spheres in the developing egg of
_Limnaea_, but he called them globules and did not recognise their
analogy with the cells of plants. According to M'Kendrick,[244] Fontana,
so far back as 1781,[245] described cells with nuclei in various tissues,
and used acids and alkalis to bring out their structure more clearly.
But it was not till 1836-7-8 that a fairly widespread occurrence of
cells in animal tissues was recognised. The pioneer in this seems to
have been Purkinje, who described cells in the choroidal plexus in
1836,[246] and compared gland cells with the cells of plants in 1837.[247]
Henle in 1837[248] and 1838[249] described various kinds of epithelial
tissue, distinguishing them according to the kind of cell composing
them; he also discovered the mode of growth of stratified epithelium.
Valentin[250] appears to have seen cells in cartilage and epithelium even
before Henle, and to have observed cells in the blastoderm of the chick.
In his report on the progress of anatomy during 1838 Johannes Mueller was
able to refer to quite a number of papers dealing with the occurrence of
cells in animal tissues. In addition to those already noted, he mentions
work by Breschet and Gluge on the cells of the umbilical cord, by
Dumortier on the cells in the liver of molluscs, by Remak and by
Purkinje on nerve cells, by Donne on the cells of the conjuctiva, cornea
and lens. He reports, too, that Turpin had compared the epithelial cells
of the vagina with the cell-tissue of plants. Mueller himself had not
only recognised the cellular nature of the notochord, but had observed
the cells of the vitreous humour, fat cells and pigment cells, and even
the nuclei of cartilage cells. From Schwann (1839) we learn that C. H.
Schults had followed back the corpuscles of the blood to their original
state of nucleated cells, and that Werneck had recognised cells in the
embryonic lens. A preliminary notice of Schwann's own work appeared in
1838 (Froriep's _Notizen_, No. 91, 1838), the full memoir in 1839, under
the title _Mikroskopische Untersuchungen ueber die Uebereinstimmung in
der Struktur und dem Wachstume der Tiere und Pflanzen_.[251]
Theodor Schwann was a pupil of Johannes Mueller, and we know that Mueller
took much interest in the new histology. It is probably to his influence
that we owe Schwann's brilliant work on the cell, which appeared just
after Schwann left Berlin for Loewen. Schwann was himself, as his later
work showed, more a physiologist than a morphologist; he did quite
fundamental work on enzymes, discovering and isolating the pepsin of the
gastric juice; he proved that yeast was not an inorganic precipitate but
a mass of living cells; he carried out experiments directed to show that
spontaneous generation does not occur. We shall see in his treatment of
the cell-theory clear indications of his physiological turn of mind.
Schwann was only twenty-nine when his master-work appeared, and the book
is clearly the work of a young man. It has the clear structure, the
logical finish, which the energy of youth imparts to its chosen work. So
the work of Rathke's prime, the _Anatomische-philosophische
Untersuchungen_ of 1832 shows more vigour and a more reasoned structure
than his later papers. Schwann's book is indeed a model of construction
and cumulative argument, and even for this reason alone justly deserves
to rank as a classic.
The first section of his book is devoted to a detailed study of the
structure and development of cartilage cells and of the cells of the
notochord, and to a comparison of these with plant cells. He accepts
Schleiden's account of the origin and development of nuclei and cells as
a standard of comparison; and he seeks to show that nucleus and
nucleolus, cell-wall and cell-contents, show the same relations and
behave in the same manner in these two types of animal cells as in the
plant-cells studied by Schleiden. The types of cell which he chose for
this comparison are the most plant-like of all animal cells, and he was
even able to point to a thickening of the cell-wall in certain cartilage
cells, analogous to the thickening which plays so important a part in
the outward modification of plant-cells. The analogy indeed in structure
and development between chorda and cartilage cells and the cells of
plants seemed to him complete. The substance of the notochord consisted
of polyhedral cells having attached to their wall an oval disc similar
in all respects to the nucleus of the plant-cell, and like it containing
one or more nucleoli. Inside the mother-cell were to be found young
developing cells of spherical shape, lacking however a nucleus.
Cartilage was even more like plant tissue. It was composed of cells,
each with its cell membrane. The cells lay close to one another,
separated only by their thickened cell-wall and the intercellular
matrix, showing thus even the general appearance of the cellular tissue
of plants. They contained a nucleus with one or two nucleoli, and the
nucleus was often resorbed, as in plants, when the cell reached its full
development. Other nuclei were in many cases present in the cell, round
which young cells could be seen to develop, in exactly the same manner
as in plants. These nuclei had accordingly the same significance as the
nuclei of plants, and deserved the same name of cytoblasts or
cell-generators. The true nucleus of the cartilage cell was probably in
the same way the original generator of the mother-cell.
Having proved the identity in structure and function of the cells of
these selected tissues with the cells of plants, as conceived by
Schleiden, Schwann had still to show that the generality of animal
tissues consisted either in their adult or in their embryonic state of
similar cells. This demonstration occupies the second and longest
section of his book.
His method is throughout genetic; he seeks to show, not so much that all
animal tissues are actually in their finished state composed of cells
and modifications of cells, as that all tissues, even the most complex,
are developed from cells analogous in structure and growth with the
cells of plants.
All animals develop from an ovum; it was his first task to discover
whether the ovum was or was not a cell. It happened that, some years
before Schwann wrote, a good deal of work had been done on the minute
structure of the ovum, particularly by Purkinje and von Baer. Purkinje
in 1825[252] discovered and described in the unfertilised egg of the fowl
a small vesicle containing granular matter, which he named the
_Keimblaeschen_ or germinal vesicle. It disappeared in the fertilised
egg. As early as 1791 Poli had seen the germinal vesicle in the eggs of
molluscs, but the first adequate account was given by Purkinje. In
1827[253] von Baer discovered the true ova of mammals and cleared up a
point which had been a stumbling block ever since the days of von Graaf,
who had described as the ova the follicles now bearing his name.[254] Even
von Graaf had noticed that the early uterine eggs were smaller than the
supposed ovarian eggs; Prevost and Dumas[255] had observed the presence in
the Graafian follicle of a minute spherical body, which, however, they
hesitated to call the ovum; it was left to von Baer to elucidate the
structure of the follicle and to prove that this small sphere was indeed
the mammalian ovum. His discovery was confirmed by Sharpey and by Allen
Thomson. Von Baer found the germinal vesicle in the eggs of frogs,
snakes, molluscs, and worms, but not in the mammalian ovum; he
considered the whole mammalian ovum to be the equivalent of the germinal
vesicle of birds--a comparison rightly questioned by Purkinje (1834). In
1834 Coste[256] discovered in the ovum of the rabbit a vesicle which he
considered to be the germinal vesicle of Purkinje; he observed that it
disappeared after fertilisation. Independently of Coste, and very little
time after him, Wharton Jones[257] found the germinal vesicle in the
mammalian ovum. Valentin in 1835,[258] Wagner in 1836,[259] and Krause in
1837,[260] added considerably to the existing knowledge of the structure
of the ovum. Wagner in his _Prodromus_ called attention to the
widespread occurrence, within the germinal vesicle of a darker speck
which he called the _Keimfleck_ or germinal spot, known sometimes as
Wagner's spot. He recognised the _Keimfleck_ in the ova of many classes
of animals from mammals to polyps. Frequently more than one _Keimfleck_
occurred.
Schwann had therefore a good deal of exact knowledge to go upon in
discussing the significance of the ovum for the cell-theory. There were
two possible interpretations. Either the ovum was a cell and the
germinal vesicle its nucleus, or else the germinal vesicle was itself a
cell within the larger cell of the ovum and the germinal spot was its
nucleus. Schwann had some difficulty in deciding which of these views to
adopt, but he finally inclined to the view that the ovum is a cell and
the germinal vesicle its nucleus, basing his opinion largely upon
observations by Wagner which tended to prove that the germinal vesicle
was formed first and the ovum subsequently formed round it. But the ovum
was not, in Schwann's view, a simple cell, for within it were contained
yolk-granules, one set apparently containing a nucleus, the others not.
Even the second set, those composing the yellow yolk, were considered by
Schwann to deserve the name of cells, because, although a nucleus could
not be observed in them, they had a definite membrane, distinct from
their contents--a conception of the cell obviously dating from the
earliest botanical notions of cells as little sacs. The yolk cells were
not mere dead food material but living units which took part in the
subsequent development of the egg. The relation between the unfertilised
egg and the blastoderm which arises from it is not made altogether clear
by Schwann. According to his account the cells of the blastoderm are
formed actually in the ovum. Round the nucleus of the egg appears a
_Niederschlag_ or precipitate which is the rudiment of the blastoderm
(p. 68). When the egg leaves the ovary the nucleus disappears, leaving
behind it this rudiment of the blastoderm, which rapidly grows and
increases in size. The blastoderm of the chick before incubation is
found to be composed of spherical anucleate bodies which Schwann
considers to be cells, because they almost certainly develop into the
cells of the incubated blastoderm, which are clearly recognisable as
such after eight hours' incubation. The serous and mucous layers can be
distinguished after sixteen hours' incubation, and it is found that the
cells of the serous layer contain definite nuclei, though such seem to
be absent in the cells of the mucous layer. Between the two layers other
cells are formed belonging to the vessel layer, which is, however, in
Schwann's opinion not a very definitely individualised layer.
Schwann's next step is a detailed demonstration of the origin of each
tissue from simple cells such as those composing the incubated
blastoderm.
"The foregoing investigation has taught us that the whole ovum shows
nothing but a continual formation and differentiation of cells, from the
moment of its appearance up to the time when, through the development of
the serous and mucous layers of the blastoderm, the foundation is given
for all the tissues subsequently appearing: we have found this common
parent of all tissues itself to consist of cells; our next task must be
to demonstrate not only in this general way that tissues originate from
cells, but also that the special formative mass of each tissue is
composed of cells, and that all tissues are either constituted by simple
cells or by one or other of the manifold kinds of modified cells" (p.
71). Five classes of tissue can be distinguished, according to the
extent and manner of the modifications which the cells composing them
have undergone. There are first of all independent and isolated cells,
such as the corpuscles of the blood and lymph, not forming a coherent
tissue in the ordinary sense. Next there are the assemblages of cells
lying in contiguity with one another, but not in any way fused; examples
of this class are the epidermal tissues and the lens of the eye. In the
third class come tissues the cells of which have fused by their walls,
but whose cell-cavities are not in continuity, such as osseous tissue
and cartilage. In the tissues of the fourth class, comprising the most
highly specialised of all, not only are the cell-walls continuous but
also the cell-cavities; to this class belong muscle, nerve and capillary
vessels. A fifth class, of rather a special nature, includes the fibrous
tissues of all kinds. This is the first classification of tissues upon a
cellular basis, and it marks the foundation of a new histology which
took the place of the "general anatomy" of Bichat. The exhaustive
account which Schwann gives of the structure and development of the
tissues in this section of his book constitutes the first systematic
treatise on histology in the modern sense, and it is still worth
reading, in spite of many errors in detail.
Schwann found it easy to demonstrate the cellular nature of the tissues
of his first three classes. With the other two classes he had more
difficulty. Fibres of all kinds, he considered, arose by an elongation
of cells, which afterwards split longitudinally into long strips,
forming as the case might be white or elastic fibrous tissue.
Muscle-fibres and nerve-fibres were formed in a totally different way,
by coalescence of cells; each separate muscle-fibre and nerve-fibre was
thus a compound cell. Capillaries, Schwann held, were formed by cells
hollowed out like drain-pipes, and set end to end--a mistaken view soon
corrected by Vogt (_Embryologie des Salmones_, p. 206, 1842).
In this detail part of his book Schwann accumulates material for a
general theory of the cell which he develops in the third and last
section. Taking up the physiological or dynamical standpoint, he points
out that one process is common to all growth and development of tissues
both in animals and plants, namely, the formation of cells, a process
which he conceives to take place in the following manner. There is,
first of all, a structureless substance, the cytoblastem, the matrix in
which all cells originate. The cytoblastem may be either inside the
cells, or, more usually, in the spaces between them. It is not a
substance of definite chemical and physical properties, for the matrix
of cartilage and the plasma of the blood alike come within the
definition. It has largely the significance of food material for the
developing cells. In plants, according to Schleiden, cells are never
formed in the intercellular substance--the cytoblastem is within the
cells; but extracellular cell formation seems to be the general rule in
animals. An intracellular formation of cells occurs only in the ovum, in
cartilage cells and chorda cells and in a few others, and even there it
is not the exclusive method of formation; a formation of cells within
cells never occurs in muscles and nerves, nor in fibrous tissue (p.
204). In the cytoblastem granules appear, which gradually increase in
size and take on the characteristic shape of nuclei; round each of these
a young cell is formed. Sometimes the young cells appear to have no
nuclei, as in the intracellular brood of chorda cells, but, as a rule, a
nucleus is clearly visible. The nucleus is indeed the most
characteristic constituent of the cell. "The most important and most
constant criterion of the existence of a cell is the presence or absence
of the nucleus," writes Schwann near the beginning of his book (p. 43).
As a general rule the nucleolus is formed first, and round it by a sort
of condensation or concretion the nucleus, which is frequently hollow,
and round this again, by a somewhat similar process, the cell. "The
whole process of the formation of a cell consists in the precipitation
round a small previously formed corpuscle (the nucleolus) of first one
layer (the nucleus) and then later round this a second layer (the cell
substance)" (p. 213). The outermost layer of the cell usually thickens
to form the membrane, but this membrane formation does not always occur,
and the membrane is not present in all cells. The nucleus is formed in
exactly the same manner as the cell, and it might with much truth itself
be called a cell--a cell of the first order, while ordinary nucleated
cells might be designated cells of the second order (p. 212). In
anucleate cells there is probably only a single process of layer
formation round an infinitely small nucleolus. In almost all nucleate
cells the nucleus is resorbed when the cell reaches its full
development, and it is larger and more important the younger the cell
is.
The cell was for Schwann not a morphological concept at all, but a
physiological; the cell was a dynamical, not a statical unit.
Cell-formation was the process at the back of all production of life,
and cells were the centres of all vital activity. Each cell was itself
an organism, and its life and activities were to some extent independent
of the lives and activities of all the other cells. The multicellular
organism was a colony of unicellular organisms, and its life was a sum
of the lives of its constituent elements. This "theory of the organism,"
which holds so important a place in biology even at the present day, is
developed by Schwann in the concluding pages of his book.
He begins by contrasting the teleological with the materialistic
conception of living things. In the teleological view, a special force
works in the living organism, guiding and directing its activities
towards a purposeful end. According to the materialistic view there are
no other forces at work in the living organism than those which act in
the inorganic realm, or at least there are none but forces at one with
these in their blindness and necessity. True, the purposiveness of
living processes cannot be denied; but its ground lies, according to
this view, not in a vital force which guides and rules the individual
life, but in the original creation and collocation of matter according
to a rational plan. The purposiveness of life is part of the
purposiveness of the universe; just as the stars circle for ever in
harmoniously adjusted paths, so do the processes of life work together
towards a common end. Both are the inevitable result of the original
distribution of matter in the primitive chaos, a distribution fixed by a
rational and foreknowing Being (p. 222).
Which of the two conceptions is to be adopted in biology? Teleological
explanations have long been banished from the physical sciences, and in
biology they are only a last resort when physical explanations have
proved incomplete (p. 223). And if the ground of the purposiveness of
living Nature is the same as the ground of the purposiveness of the
universe, is it not reasonable to suppose that explanations which have
proved satisfactory for inorganic things will in time with sufficient
knowledge prove adequate also for organic things?
The teleological conception, again, leads to difficulties particularly
when it is applied to the facts of reproduction. If we suppose that a
vital force unifies and coordinates the organism and is its very
essence, we must also suppose that this force is divisible and that a
part of it--separated in reproduction--can bring about the same results
as the whole. If on the contrary the forces having play in the organism
are the mere result of the particular combination of the matter
composing it, the reconstruction of a particular combination of
molecules in the ovum is all that is necessary to set development
a-going along exactly the course taken by the ovum of the parent.
Another argument against the teleological view is derived from the facts
of the cell-theory. The cell-theory tells us that the molecules of the
living body are not immediately built up in manifold combinations to
form the organism, but are formed first into unit-constructions or
cells, and that these units of composition are invariably formed in all
development, of plants and animals alike, however diverse the goal of
development may be. If there were a vital principle would we not expect
to find that, scorning this roundabout way of reaching its goal, it went
straight to the mark, taking a different and distinctive course for each
individual development, building up the organism direct without the
intermediary of cells? But since there is a universal principle of
development, namely, the formation of cells, does it not seem that the
cells must be the true organisms, that the whole "individual" organism
must be an aggregate of cells, and that the concept of individuality
applied to the organism is accordingly a logical fiction? And it is just
upon this notion of the individuality of the organism that the
teleological concept is based. The teleological view can perhaps not be
completely refuted until the adequacy of materialistic explanations has
been finally shown; but it is certain that the most promising method for
research is the materialistic (p. 226).
"We start out then from the assumption that the basis of the organism is
not a force acting according to a definite plan; on the contrary, the
organism arises through the action of blind and necessary laws, of
forces which are as much implicit in matter as those of the inorganic
world. Since the chemical elements in organic Nature differ in no way
from those of inorganic Nature, the ground or cause of organic phenomena
can consist only in a different mode of combination of matter, either in
a peculiar mode of combination of the elementary atoms to form atoms of
the second order, or in the particular arrangement of these compound
molecules to form the separate morphological units of the organism or
the whole organism itself" (p. 226). Accepting then the materialistic
conception of the organism, we have to consider this further problem.
Does the ground of organic processes lie in the whole organism or in its
elementary parts? Translated into terms of metabolism--note the
physiological point of view--the question runs, are metabolic processes
the result of the molecular construction of the organism as a whole, or
does the centre of metabolic activity lie in the cell? Is it the cell
rather than the organism that is the immediate agent of assimilatory
processes? In the first alternative the cause of the growth of the
constituent parts lies in the totality of the organism; in the other
alternative:--"Growth is not the result of a force having its ground in
the organism as a whole, but each of the elementary parts possesses a
force of its own, a life of its own, if you will; that is to say, in
each elementary part the molecules are so combined as to set free a
force whereby the cell is enabled to attract new molecules and so to
grow, and the whole organism exists only through the reciprocal action
of the single elementary parts.... In this eventuality it is the
elementary parts that form the active element in nutrition, and the
totality of the organism can be indeed a condition, but on this view it
cannot be a cause" (p. 227).
To help in the decision of this question, appeal must be made to the
facts established as to the cellular nature of the organism and of its
reproductive elements. We know that every organism is composed of cells,
which are formed and grow according to the same laws wherever they are
found, whose formation therefore is everywhere due to the same forces.
If we find that certain of these cells--all of which we know to be
essentially identical one with another--have the power when separated
from the others of growing and developing into new organisms, we can
infer that not only such cells but also all other cells have this
assimilatory power. The ova of animals, the spores of plants, the
isolated cells of lower organisms in general, all show the power of
separate assimilation and development. "We must therefore, in general,
ascribe to the cell an individual life, that is to say, the combination
of the molecules in the single cell does suffice to produce the force
whereby the cell is enabled to draw to itself new molecules. The ground
of nutrition and growth lies not in the organism as a whole, but in the
separate elementary parts, the cells. The fact that it is not every cell
that can continue to grow when separated from the organism is not in
itself an objection to this theory, any more than it is an objection to
the individual life of a bee that it cannot continue to exist apart from
the swarm. The activation of the forces existing within the cell depends
on conditions which the cell encounters only in connection with the
whole" (pp. 228-9).
Schwann's next step is to discover what are the essential forces active
in the cell, and here he enters the realm of hypothesis. He finds they
can be reduced to two--an attractive force and a metabolic force. The
attractive force is seen in the process of cell-formation, where first
of all the nucleolus is formed by a concentration and precipitation of
substances found free in the cytoblastem, and in the same way the
nucleus and later the cell are laid down as concentric precipitates from
the cytoblastem. Cell-formation also involves the second or metabolic
force, by means of which the cell alters the chemical composition of the
medium surrounding it so as to prepare it for assimilation. Schwann's
attractive force brings about the actual taking up of the prepared
substance; his metabolic force is the cause of the digestion of food
substances, and is nearly identical with enzyme action. With what
inorganic process, he now asks (p. 239), can the process of
cell-formation be most nearly compared, and the answer obviously is,
with the process of crystallisation. Cells are, it is true, quite
different in shape and consistency from crystals, and they grow by
intussusception, not by apposition--their plastic or attractive forces
seem therefore to be different. A still more important difference is
that the metabolic force is peculiar to the cell. Yet there are
important analogies between crystals and cells. They agree in the
important respect that they both grow in solutions at the cost of the
dissolved substance, according to definite laws, and develop a definite
and characteristic shape. It might even be maintained, Schwann thinks,
that the attractive force of crystals is really identical with that of
cells, and that the difference in result is due merely to the difference
between the substance of the cell and the substance of the crystal. He
points out how organic bodies are remarkable for their powers of
imbibition, and he seeks to show that the cell is the form under which a
body capable of imbibition must necessarily crystallise, and that the
organism is an aggregate of such imbibition-crystals. The analogy
between crystallisation and cell-formation he works out in the following
manner:--"The substance of which cells are composed possesses the power
of chemically transforming the substance with which it is in immediate
contact, in somewhat the same way as the well-known preparation of
platinum changes alcohol into acetic acid. Each part of the cell
possesses this property. If now the cytoblastem is altered by an already
formed cell in such a way that a substance is formed that cannot become
part of the cell, it crystallises out first as the nucleolus of a new
cell. This in its turn alters the composition of the cytoblastem. A part
of the transfomed substance may remain in solution in the cytoblastem or
may crystallise out as the beginning of a new cell; another part, the
cell-substance, crystallises round the nucleolus. The cell-substance is
either soluble in the cytoblastem and crystallises out only when the
latter is saturated with it, or it is insoluble and crystallises as soon
as it is formed, according to the aforementioned laws of the
crystallisation of imbibition-bodies; it forms thus one or more layers
round the nucleolus, etc. If one imagines cell-formation to take place
in this way, one is led to think of the plastic force of the cell as
identical with the force by means of which a crystal grows" (pp.
249-50).
Two difficulties have to be faced by this theory--(1) the origin of the
metabolic power of the cells, (2) the reason why the cells arrange
themselves so as to form an organism of complex and definite structure.
Schwann tries to explain the origin of the "metabolic" action, the
analogy of which with the contact-action of colloidal platinum he
recognises, by attributing it to the peculiar structural arrangements of
molecules. In attempting to account for the harmonious structure of the
organism he points to the analogy of ordinary crystals, which often form
complex and regular tree-like arrangements; plants in particular
resemble these regularly shaped crystal-aggregates.
The whole ingenious theory is offered merely as an hypothesis and a
guide to research. It is interesting as one of the most carefully
thought-out attempts ever made to give a thorough-going materialistic
account of the origin and development of organic form, and it arose
directly out of the cell-theory.
Schleiden and Schwann started out from an erroneous theory of the origin
and development of cells, which impaired to some extent the value of
their results. It was not long, however, before their theory of the
origin of cells by "crystallisation" from an intra- or extra-cellular
cytoblastem was challenged and overthrown, and the generalisation that
cells originate by division from pre-existing cells put in its place.
This was established for plant cells by Meyen, Unger, von Mohl, Naegeli
and Hofmeister in or about the forties.[261] Criticism of the
Schwann-Schleiden theory from the zoological side was suggested by the
study of the segmentation of the ovum--the developmental process in
which the multiplication of cells is most easily observed. The
segmentation of the ovum was well known to Schwann, for the process had
been described in the frog by Prevost and Dumas in 1824,[262] in the frog
and newt by Rusconi,[263] and an elaborate study of the process in the
frog had been made by von Baer.[264] Schwann indeed suspected that there
must be some connection between the segmentation of the ovum and the
formation of cells, but he did not realise that the cellular blastoderm
of the chick was formed by the division or segmentation of the egg-cell.
Segmentation was soon found to be of widespread occurrence. Von Siebold
in 1837 described the process in Entozoa,[265] and in the same year Loven
saw segmentation in _Campanularia_,[266] and Sars in the starfish and in
Nudibranchs.[267]
In 1838 Bischoff[268] observed segmentation in the mammalian ovum, and the
whole course of segmentation in the ovum of the rabbit from the 2-celled
to the morula stage was carefully described and figured by Barry[269] in
1839. C. Vogt[270] in 1842 described segmentation in _Coregonus_ and
_Alytes_. The discovery of segmentation in the ovum of birds was not
made until 1847, by Bergmann,[271] confirmed independently by Coste[272]
in 1850. By 1848 segmentation had been noted in _Hydra_ and various
hydroids, in acalephs, in starfish, polyzoa, nematodes, rotifers,
leeches, oligochaetes, polychaetes, in most groups of molluscs and
arthropods, and in all the vertebrate classes.[273]
The process was at first held to be merely one of yolk-division, or
_Dotterfurchung_, and its details were by most interpreted in the light
of the Schleiden-Schwann theory of cell-formation.
The first steps towards a truer conception of the process seem to have
been taken by Bergmann, who in 1841[274] called attention to the presence
of nuclei in the segmentation-spheres of the frog's egg, and by Bagge in
the same year, who observed that division of the nuclei preceded the
multiplication of the segmentation spheres.[275] He considered the nuclei
to be anucleate cells, and the same view was taken by Koelliker in
1843.[276] Next year, however, in his classical paper on Cephalopod
development[277] Koelliker came to the opinion that they were really
nuclei. He showed that segmentation was brought about by cell-division,
that between "total" and "partial" segmentation there was a difference
of degree and not of kind, and that the cells of the body were formed by
division of the segmentation spheres. He held, however, that the nuclei
multiplied endogenously and not by division. The division of nuclei was
observed by Coste in 1846.[278] Leydig in 1848[279] took the necessary step
in advance and maintained that the nuclei as well as the cells increased
always by division. He was supported by Remak, who in a paper of
1852,[280] and more fully in his monumental _Untersuchungen ueber die
Entwickelung der Wirbelthiere_ (Berlin, 1850-55), proved that in the
frog's egg at least segmentation was a simple process of cell-division,
initiated always by division of the nucleus.[281]
One point Remak left undecided--the fate of the _Keimblaeschen_ or
egg-nucleus. It was generally held, even so late as the 'fifties, that
the egg-nucleus disappeared just before segmentation began--Bischoff
clung to this belief even in 1877.[282] Though Barry had held in 1839 that
the egg-nucleus does not disappear in segmentation, J. Mueller seems to
have been the first actually to prove that it forms by division the
nuclei of the first two segmentation spheres. He furnished the
demonstration in the egg of _Entoconcha mirabilis_,[283] and his paper was
known to Remak, who could not, however, observe a similar division of
the egg-nucleus in the frog. Mueller's discovery was confirmed for
_Oceania armata_ by Gegenbaur,[284] and for _Notommata sieboldii_ by
Leydig.[285]
In 1854 Virchow,[286] previously a supporter of Schwann, crystallised the
new views in the famous phrase--_Omnis cellula e cellula_--and gave wide
publicity to them in his classical lectures on Cellular Pathology,
delivered in 1858.[287] The new doctrine of cell-formation was also taught
by Leydig[7] in his text-book of histology, published in 1857.
The Schleiden-Schwann theory of the origin of cells by generation in a
cytoblastem was now definitely overthrown.
The importance of the protoplasmic content of the cell was brought into
prominence through the work of Dujardin,[289] Purkinje,[290] Cohen[291] and
Max Schultze.[292] The last-named in 1861 proposed a definition of the
cell which might be accepted at the present day. "A cell," he wrote, "is
a little blob of protoplasm containing a nucleus" (p. 11).
[238] _Theoria generationis_, Halae, 1759.
[239] See J. v. Sachs, _Geschichte der Botanik_, book ii.,
Eng. Trans., 2nd impr., 1906.
[240] Mueller's _Archiv_, pp. 137-76, 1838.
[241] _Trans. Linnean Soc._, xvi., p. 710, 1833.
[242] _Myxinoiden_, i. Theil., p. 89, 1835.
[243] _Ann. Sci. nat._ (2) (_Zool._) ii., pp. 107-18, pl.
11, 1834.
[244] _Proc. Phil. Soc. Glasgow_, xix., pp. 71-125,
1887-8.
[245] _Traite sur le venin de la vipere_, 1781.
[246] Mueller's _Archiv_, 1836.
[247] J. Mueller, _Jahresbericht ue. d. Fortschritte der
anat.-physiol. Wissenschaften im Jahre_ 1838. Mueller's
_Archiv_, 1838.
[248] _Symbolae ad anatomiam villorum imprimis eorum
epithelii_, Berlin, 1837.
[249] _U. d. Ausbreitung des Epitheliums im menschlichen
Koerper_. Mueller's _Archiv_, 1838.
[250] See Schwann's _Bemerkungen_ at the end of his
_Mikroskopische Untersuchungen_.
[251] Republished in Ostwald's _Klassiker der exakten
Wissenschaften_, No. 176, Leipzig, 1910. References in
the text are to the original pagination.
[252] _Symbolae ad ovi avium historiam_.
[253] _De ovi mammalium et hominis genesi_.
[254] _De mulierum organis_, 1672.
[255] _Ann. Sci. nat._, iii., p. 135, 1842.
[256] _Recherches sur la generation des Mammiferes_.
Report by Academy Committee. _Ann. Sci. nat._ (2)
(_Zool._) ii., pp. 1-18, 1834; also _Embryogenie
comparee_, 1837.
[257] _Lond. and Edin. Phil. Mag._ (3) vii., 1835; _Phil.
Trans._ 1837.
[258] _Handbuch der Enfwickelungsgeschichte_, 1835, and
Mueller's _Archiv_, 1836.
[259] _Prodromus historiae generationis hominis atque
animalium_, Lipsiae, 1836.
[260] Mueller's _Archiv_, 1837.
[261] Sachs, _History of Botany_, Book ii.
[262] _Ann. Sci. nat._, i., pp. 110-14, 1824. Swammerdam
is said to have observed the 2-celled stage in the egg
of the frog (_Bibl. Nat._, 1752), and Roesel v. Rosenhof
the same stage in the tree-frog (_Hist. nat. ranarum
nostratium_, 1758).
[263] _Developpement de la grenouille commune_, Milan,
1826. _Biblioteca italiana_, lxxix., 1836, and Mueller's
_Archiv_, 1836. Agassiz is said by Vogt (1842) to have
seen segmentation in the Perch as early as 1831.
[264] Mueller's _Archiv_, 1836.
[265] In Burdach, _Die Physiologie als
Erfahrungswissenschaft_, 2nd Ed., vol. ii.
[266] Wiegmann's _Archiv_, 1837.
[267] _Bericht Versamml. deutsch. Naturf. in Prag_, 1837.
[268] _Bericht Versamm. deutsch. Naturf. in Freiburg_,
1838. Later in his _Entw. d. Wirbelth_., and in his
papers on the development of the rabbit.
[269] _Phil. Trans._, 1839. See particularly Pl. vi.,
figs. 105-12.
[270] _Embryologie des Salmones_ 1842.
[271] Mueller's _Archiv_, 1847.
[272] _C.R. Acad. Sci._, xxx., p. 638.
[273] See review by Leydig in _Isis_, 1848, pp. 161-193.
[274] Mueller's _Archiv_, pp. 89-102, 1841.
[275] _De evolution Stronzyli auric. el Ascaridis acum._,
Erlangen, 1841.
[276] Mueller's _Archiv_, pp. 66-141, 1843.
[277] _Entwickelungsgeschichte der Cephalopoden_, Zurich,
1844.
[278] _Froriep's Notizen_, No. 800, 1846.
[279] _Isis_, 1848.
[280] Mueller's _Archiv_, p. 47, 1852, also 1854 and 1858.
[281] See particularly Plate IX., figs. 3-7.
[282] _Hist.-krit. Bemerkungen zu den neuesten
Mittheilungen ue. d. erste Entwickelung d.
Saeugethiereier_, Muenchen, 1877.
[283] _Monatsber. Akad. Wiss. Berlin_, 1851.
[284] _Zur Lehre von Generationswechsel u. d. Fortpflanzen
d. Medusen u. Polypen_.
[285] _U. d. Bau u. d. system. Stellung d. Raederthiere_,
1854.
[286] _Arch f. path. Anat. Phys._, vii., pp. 1-39, 1854.
Also in his _Beitraege z. spec. Path. u. Therapie_.
[287] _Die Cellularpathologie_, Berlin, 1858.
[288] _Lehrbuch der Histologie_, 1857.
[289] _Ann, Sci. nat._ (2) iii., pp. 108-9 and pp. 312-4,
1835. Also iv, pp. 343-77.
[290] 1839 or 1840.
[2913] _Nova Acta Acad. Leop._, xxii., 1850. Trans. in 1853
for Ray Society.
[292] _Arch. f. Anat. u. Physiol._, pp. 1-27, 1861.