THEORIES OF DEVELOPMENT
The experimental work that Pflüger carried out in 1883 on the effect of
gravity on the cleavage of the frog’s egg, and the conclusions that he
drew from his experiments, mark the starting-point for the modern study
of experimental embryology.[124] We can trace the influence of Pflüger’s
results through most of the more recent work, and one of the conclusions
reached by Pflüger, namely, that the material of the egg may be divided
by the cleavage planes in any way whatsoever without thereby altering
the position of the embryo on the egg, is, I think, one of the most
important results that has yet been reached in connection with the
experimental work on the egg. Pflüger’s analysis of the factors that
direct the development has also an important bearing on the
interpretation of the development of a whole embryo from a part of an
egg.
Pflüger found that in whatever position the frog’s egg is turned before
it begins to divide, the first two planes come in vertically, and the
third horizontally, and that later the smallest cells are always formed
in the upper hemisphere. He concluded, therefore, that gravity has some
sort of influence in determining the position of the planes of cleavage.
Pflüger observed that the position of the median plane of the body of
embryos that have developed from eggs turned into unusual positions does
not, as a rule, correspond to the plane of the first cleavage, but that
the embryo generally lies on that meridian of the egg that passes
through the primary egg axis and the highest point of the egg in its new
position. Since any meridian may happen to be placed uppermost, the
embryo may, therefore, develop upon any one of the primary meridians,
and hence the material must be isotropous around the primary axis.
Furthermore, since the embryo appears always below the middle of the
egg, in whatever position the egg may lie, we must conclude that in each
meridian the material is also isotropic.
It may be pointed out that while more recent work has substantiated, on
the whole, the latter conclusions[125] of Pflüger, just stated, still
the results of studies of regenerative phenomena of organisms show that
the conclusions are not necessarily the only ones deducible from the
experiments; for, although it may be true that any possible primary
meridian of the egg may become the median plane of the body of the
embryo, it does not follow that there is no one organized plane always
present in the normal egg, _i.e._ the egg may not be entirely isotropic.
That this may be the case is shown in the regeneration of pieces of
adult animals in which a piece cut to one side of the old median plane
may develop a new plane of symmetry of its own. This possibility must be
also admitted for the egg. If we substitute the term “totipotence,”
meaning that any meridian of the egg has the possibility of becoming the
median plane of the embryo, in place of Pflüger’s term “isotropy,” we
remove this element of possible error from his statement.
Roux and Born have shown that the only action that gravity has on the
frog’s egg is to bring about a rearrangement of the contents of the egg,
a phenomenon that Pflüger had not observed. The lighter part flows to
the highest region of the egg, and the heaviest to the bottom of the
egg, hence the change in the position of the cleavage planes observed by
Pflüger that begin in the upper, more protoplasmic part of the egg.
Another series of experiments, that we also owe, in the first place, to
Pflüger (’84), consist in compressing the egg before and during its
cleavage. The position of several of the cleavage planes may be altered,
yet a normal embryo develops from the egg. The same experiment has been
repeated by Hertwig (’93), and by Born (’93), on the frog’s egg, and by
Driesch (’92), Ziegler (’94), myself (’93), and others, on the egg of
the sea-urchin, with substantially the same results. The value of the
experiment lies not so much in showing that the coincidence between the
first cleavage planes and the orienting planes of the body may be lost,
as in showing that under these circumstances the nuclei have a different
distribution in the protoplasm from that which they hold in the normal
egg. Any theory of development that depends on the qualitative
distribution of nuclear products during the cleavage period meets with
great difficulties in the light of these results, and in order to
overcome them will be obliged to add qualifications of such a kind as
materially to alter its simplicity. Roux’s theory, for instance, comes
into this category. Roux (’83) suggested that since the complicated
karyokinetic division of the nucleus is carried out in such a way as to
insure a precise division of the chromatin, and since the qualities of
the male are transmitted to the egg through the chromatin of the
spermatozoon, it is probable that the division of the chromatin is a
qualitative process, by means of which the elements are distributed to
different parts of the egg. According to Roux, the first division of
the frog’s egg divides the material of the right half of the embryo from
that of the left; the second division separates the material of the
anterior half from that of the posterior half. Roux limited, to a
certain extent, his hypothesis to these two divisions of the frog’s egg,
and stated further that it is not improbable that during the later
stages of development there may take place an interaction of the parts
on each other, and this interaction would be another factor in the
development. Weismann has adopted Roux’s hypothesis, and has extended it
to all organisms, and to most of the divisions of the developing egg, at
least to all those divisions in which the qualities of the layers,
tissues, organs, etc., are separated. On this slight basis he has
constructed his theory of development and of regeneration. It is
important, therefore, to examine critically the evidence furnished by
experimental embryology for or against this hypothesis of a qualitative
division of the egg during the cleavage period.
The development of a half embryo from one of the first two blastomeres
of the frog’s egg, in Roux’s experiment, seemed to support Roux’s
hypothesis, but it was not long before it was seen that the presence of
the other blastomere vitiated the evidence to such an extent as to
render it worthless, so far as this hypothesis is concerned. Then
followed the experiments with the isolated blastomeres of the
sea-urchin, amphioxus, jelly-fish, teleost, ascidian, triton, etc., in
which each blastomere, when completely separated, gives rise to a whole
embryo. From these experiments Driesch and Hertwig drew the opposite
conclusion, namely, that during the cleavage there is a quantitative
division of the egg into blastomeres that are equivalent, or at least
totipotent. Roux attempted to meet the results of these experiments in
two ways. He pointed out that in several of these cases the isolated
blastomere divides as a half or as a fourth of the egg, and that in the
sea-urchin this leads to the formation of an open half-blastula. In the
second place, Roux brought more to the front his subsidiary hypothesis
of the reserve germ plasm. He supposed that along with the early
qualitative division of the nucleus, by means of which each part
receives its particular chromatic substance, there is also a
quantitative division of a sort of reserve germ plasm contained in the
nucleus. Each cell _may_ receive also a part of this material, and hence
each cell may contain the potentialities of the whole egg. This reserve
plasm may be awakened by any change that alters the normal development,
as, for instance, when the blastomeres are separated. It may take some
time for this reserve stuff to wake up, as shown by the half-development
of the sea-urchin’s egg that goes on for some time after the separation
of the blastomeres. This hypothesis cannot be objected to on purely
formal grounds, but we are not so much concerned with a purely logical
hypothesis as with a verifiable one.
It has been pointed out that the experiment of compressing the egg in
different planes that leads to a new distribution of the nuclei is a
formidable obstacle to Roux’s hypothesis. If the nuclear divisions in
the compressed egg are of the same sort as in the normal egg, we should
expect to find as a result either a monstrous form with all its parts
misplaced, or, if the parts are mutually dependent, nothing at all. Roux
has attempted to meet this case by supposing that the nucleus itself
responds to the change in the protoplasm and alters its divisions in
such a way as to send to each part of the compressed egg the right sort
of material for that part. This means that the nucleus can so entirely
change the sequence of its divisions that instead, for instance, of
sending to each pole of the first spindle the material of the right and
left sides of the body, as it does normally, it may divide under
compression in such a way that the material for the anterior half of the
embryo is separated from that of the posterior half. That a change
involving such a vast number of qualities could take place, as a result
of the slight compression on the egg that brings about a change in the
position of the spindle, seems highly improbable. It is, of course, not
a disproof of the hypothesis to show that it involves very great
complications, for the very assumption itself of a qualitative division
of the nucleus, in the Roux-Weismann sense, involves us in great
complications.
A more damaging criticism of the hypothesis of a qualitative division of
the nucleus is found in an appeal to direct observation, which shows
that the chromatin divides always into exactly equal parts. In many
cases we know, from the subsequent fate of the cells, that two cells
arising from the same cell play very different rôles in the subsequent
development, yet the chromatin of the nucleus is always divided equally.
The development of the isolated blastomeres of the ctenophore egg may
seem at first sight to give support to Roux’s hypothesis, for in this
case the first two cells are completely separated, and yet give rise to
half-structures. Crampton’s experiments on the eggs of ilyanassa may
also appear to be evidence in favor of this view. In fact, however, they
give no more support to the idea of a qualitative division of the
nucleus than they do to that of a qualitative division in the
protoplasm, and there are some further experiments on the ctenophore egg
which indicate that it is the latter rather than the former sort of
division that takes place. As stated in the preceding chapter, Driesch
and Morgan found that, if a part of the protoplasm of the unsegmented
egg of the ctenophore is removed, an incomplete embryo develops,
although the whole of the segmentation nucleus is present. Ziegler’s
results show that, even after the removal of that part of the egg from
which the micromeres develop, the segmentation may still be like that of
the whole egg, and this shows that the egg has great powers of
recuperation (at least in a symmetrical plane), so far as its protoplasm
is concerned. If, however, it is true that when a part is cut off
unsymmetrically the protoplasm cannot reorganize itself, then the
conclusion that Driesch and Morgan drew in regard to the protoplasm will
hold, provided, as seems to be the case, the smaller blastomere of the
first two is large enough to produce the typical structures. The main
point is this: If the protoplasm readjusts itself after the operation,
so that the piece divides as a whole, a complete embryo develops; if,
however, the protoplasm does not readjust itself, and the piece divides
as a part, an incomplete embryo is formed. Since in both cases the same
nucleus is present, and since the difference is obviously connected with
a change in the protoplasm, it seems much more probable that the
phenomenon of whole and half development is connected with the
protoplasm and not with the nucleus.
The hypothesis that Pflüger, Hertwig, and Driesch have adopted, namely,
that the cleavage divides the egg into potentially equal parts, stands
in sharp contrast to the Roux-Weismann conception of development. There
are two ideas in the former view which should be kept, I think, clearly
apart: the first is, that the blastomeres are potentially equal
(isotropous), because they are exactly alike; the second is, that
despite the differences that may exist amongst them they are still
potentially able to do the same thing, _i.e._ they are totipotent. The
former alternative is that adopted by Pflüger, Hertwig, and Driesch; the
latter view, to which Driesch seems more inclined in his later writings,
is the one that I should prefer.[126] The first four blastomeres of the
sea-urchin’s egg appear to be exactly alike, and we find that each can
make a whole embryo. If we assume, however, that despite their likeness
and their totipotence they are different in so far as there is present
in the protoplasm a bilateral structure, we are nearer, in my opinion,
to the truth; for, unless we assume the bilateral structure to be
determined later by some external factor, of which there is no evidence,
we must suppose that after fertilization, at least, there must be a
bilateral structure to the protoplasm, and this view is borne out in one
sense by the subsequent mode of cleavage of the blastomeres if they are
separated. Whether this bilaterality of the fertilized egg leads to the
bilaterality of the cleavage is, however, a different question. In some
cases this _appears_ to be the case, in others it is clearly not the
case, and we must suppose that some other condition determines the
bilaterality of the later stages than that which influences the
cleavage. Many facts of experimental embryology and of regeneration
show, moreover, that a new bilateral structure may be readily assumed by
pieces that have lost their connection with the rest of the organism.
After the third division of the egg of the sea-urchin, four of the
blastomeres are somewhat different, so far at least as the material of
which they are made up is concerned, from the other four; yet any one of
the eight blastomeres, or groups of blastomeres, can produce a whole
embryo. The same statement can be made for much later stages, since it
has been found that fragments from any part of the blastula wall can
give rise to whole embryos, and we may safely attribute this property to
all the cells, although on account of the size of the cells of later
stages they cannot individually produce a whole embryo, but each can
produce any part of an embryo, which amounts to the same thing. If we
assume that all of these cells are exactly alike, as Hertwig has done,
we fail to see how the next stage in the development could take place,
unless some external factor could act in such a way as to change the
different parts of the egg. We have, however, no reason to suppose that
all the cells are alike because they are all potentially equal. Even
pieces of an adult animal--of hydra or of stentor, for example--can
produce new whole organisms, although we must suppose these pieces to be
at first as unlike as are the parts of the body from which they arise.
Moreover, we do not know of a single egg or embryo in which we cannot
readily detect differences in different parts of the protoplasm.
Can these gross differences, that we can see, in the materials of the
egg explain the different development of the parts of the egg? It can be
shown, I think, that they do not _necessarily_ determine the result. If
we cut in two a blastula, so that one piece contains only the cells from
the animal half and the other piece cells from the vegetative half, each
produces a whole embryo; yet the one half lacked just those parts which
by hypothesis were supposed to determine the gastrulation of the other
half. If we suppose that the materials or structures that are
characteristic of the vegetative half are gradually distributed from the
vegetative to the animal pole in decreasing amounts, then any piece of
the egg will contain more of these things at one pole than at the other.
If, then, it could be shown that the gastrulation depends on the
relative amounts of these materials in the different parts of the
blastula, the difficulty met with in the former view disappears in part.
I say in part, because the relative amount of materials that produces
the results implies a connecting substratum that is acted upon and
determines the result. Even if we suppose that this polar distribution
of material could account for the polar invagination, we should still be
at a loss to account for the origin of the bilateral symmetry. In many
eggs there is no evidence of a bilateral distribution of the material,
although in some few cases there may be, so far as the form is
concerned, a plane of bilateral symmetry. But even if it is supposed to
be present in all eggs, and to coincide with the first plane of cleavage
(or with any other cleavage plane), we still could not explain the
bilateral symmetry of the one-half and one-fourth whole embryos that
come from the corresponding isolated blastomeres. If a preëxisting
bilateral plane exists in the egg, it must be reëstablished in some way
in the isolated blastomere and in pieces of the blastula wall. In the
latter case this could scarcely be brought about by a redistribution of
the gross contents of the piece, since the presence of cell walls would
interfere with such a process.
This analysis shows, I think, that the transformation of a piece into a
new whole really involves a change in the fundamental structure itself.
There cannot be much doubt that both the polarity and the bilaterality
of the egg, or of a piece of the egg, belong fundamentally to the same
class of phenomena, and we are forced to the supposition that they are
inherent peculiarities of the living substance. Driesch thought at one
time that it is only necessary to suppose that the protoplasm, and every
part of it, possesses a primary polarity, and that some inequality in
the material might determine the plane of bilaterality; but later he
thought it necessary to assume also the presence of a bilateral
structure in the protoplasm, and in all parts of it. This assumption of
every part having a polar and a bilateral structure, and the polarity
and bilaterality of the whole being the sum total of those of all its
parts, is, I think, insufficient to meet the situation. If, for example,
the first plane of cleavage coincides with the median plane of the body,
the right blastomere has a structure that leads to the formation of the
right side of the body, and similarly for the left blastomere. If the
two blastomeres are separated, and each gives rise to a whole embryo
with a new plane of bilateral symmetry, we must suppose that a new
bilaterality has been produced. It does not make the problem any simpler
to assume, as Driesch has done, that this is brought about by the
elements rearranging themselves bilaterally on each side of a new plane
that passes through the middle of the isolated blastomere, for what we
need to have explained is what determines the new median plane. It seems
to me that the problem is not any simpler, if we assume the polarity and
bilaterality to be the property of a large number of elements, as
Driesch has done, than if we assume at once the polarity and
bilaterality as characteristic of the whole egg. The difficulty of
understanding how a new bilaterality can be induced in a piece of the
whole is as great on the one assumption as on the other. Not only is
it, I think, a much simpler idea to suppose the structure is something
pertaining to the whole and is not the sum total of smaller wholes, but
the idea is more in accord with other phenomena.
We meet here, I think, with precisely the same problem that we meet with
in the regeneration of parts of adult animals. If a planarian is cut in
two lengthwise, along the middle line, each half produces new tissue at
the cut-side, out of which the missing half is formed. In this case the
old median plane remains, more or less, as the median plane of the new
worm, _i.e._ the structure of the new part is built up on that of the
old. Very much the same result follows when the worm is cut
longitudinally into two unequal parts. The larger piece retains its old
plane of symmetry and adds to the cut-edge a new part that completes the
symmetry. The smaller piece also builds up new material along the
cut-edge, and a new plane of symmetry is formed between the old and the
new parts. Here, also, a median plane is established at the edge of the
old material, but in this case the material lay to one side of the old
middle line, and this involves the changing over to a large extent of
the old material, so that it fits in with the new structures of the new
median plane.
In those forms in which the readjustment takes place entirely in the old
part, the change of conditions is more difficult to interpret. In some
respects hydra gives us an intermediate condition, but since it is a
radially symmetrical instead of bilaterally symmetrical form, the
transformation is not so obvious. If a cylindrical piece is cut from the
body, and is then cut lengthwise into two half-cylinders, each closes in
and makes a cylinder of smaller diameter. A little new tissue may appear
along the fused edges, but the missing half is not replaced, and a new
hydra with a body of half size is formed from the piece. It is to all
appearances a radially symmetrical form, and we must think, in this
case, of the new axis of symmetry as having shifted to the middle of the
piece. As yet no similar experiments have been made on a bilateral
animal that regenerates by morphallaxis, so that we have nothing to
appeal to for comparison with the bilateral egg, but the results, just
described for the planarian and for hydra, indicate how a change might
take place in pieces of adult animals that would lead to the formation
in them of a new symmetrical structure. If we imagine a case of this
sort, and suppose that after separating a piece from the side the
cut-edge closed in and the piece assumed a symmetrical form, it is
conceivable that a new plane of bilateral symmetry might soon appear in
the middle of the piece owing to the symmetrical form of the piece; or
the new plane of symmetry might slowly shift from the cut-edge toward
the middle of the piece, after reaching which the balance or equilibrium
would be attained. This statement, it must be confessed, is little more
than a supposition, and rests on the unproven assumption that the
internal symmetry may develop in response to a symmetrical change in
shape of the piece as a whole, which is partly the outcome of purely
physical factors. At present, however, I see no other probable inference
from the facts.
If we suppose a bilateral structure is present in the fertilized egg,
and that it corresponds to the first plane of cleavage, a change of the
sort that we have just sketched above may be supposed to take place when
the blastomeres are separated. The stimulus is found in the new
spherical form assumed by the isolated blastomere, and we may imagine
the change to take place, in the way indicated, by virtue of the old
bilaterality that is present, the change beginning at the side
originally in contact with the other half.
There are several facts which seem to indicate that a change in the
axial relations of the egg is very easily brought about before any
definite organs have appeared. The fact that the point of entrance of
the spermatozoon in the egg of the frog[127] and of the sea-urchin[128]
may determine the first plane of cleavage points to this conclusion. The
fact that, in the frog, and also in the triton, the median plane of the
embryo corresponds sometimes to the first, sometimes to the second plane
of cleavage, and sometimes to neither one, shows that the bilaterality
of the embryo-structure may or may not coincide with the plane of
cleavage. In the fish also there seems to be no correspondence between
the planes of cleavage and those of the embryo, so that different
factors may determine the two. We should not be justified in concluding
from this evidence that a bilateral structure is absent, but rather that
it is of such a sort as to be independent of the cleavage, and that it
can be also easily changed. It is probable that the kind of organization
that we must suppose to exist in the egg is of a very simple sort, and
capable of easy readjustment. There is certainly no evidence in favor of
the view that the organization of the egg need be anything like the
organization of the embryo that comes from the egg, although the
organization of the egg may be perfectly definite in its character.
Until we know more of the nature of this organization, it is useless to
speculate further as to how it can be altered.
Another question of much importance in connection with our present topic
is the part played by the individual cells in the early development of
the whole egg, or of any part of the egg. Hertwig (’93) thinks that the
development is brought about by the action of the individual cells on
each other. Driesch, when he states that the fate of a blastomere is a
function of its position in the whole, does not commit himself
definitely one way or the other so far as the cell as a unit is
concerned. Whitman and others have urged the insufficiency of the cell
theory, and think that cell boundaries play no important part in the
development, but that the embryo develops as a whole. This has seemed to
me to be the more probable view in the light of certain results of
experimental embryology. Driesch, in later papers, has also opposed
Hertwig’s idea, and Wilson in his book on _The Cell_ has also, to a
certain extent, adopted this point of view. The formation of a typical
larva in the sea-urchin and in amphioxus out of one-half or one-fourth
the whole number of cells demonstrates, I think, the insufficiency of
the cell-unit hypothesis. The discovery of continuous protoplasmic
connections between neighboring cells, and the formation of new
protoplasmic connections between all regions, as found by Mrs. G. F.
Andrews,[129] gives us a basis of fact on which to rest the hypothesis
of the embryo being a whole structure. This view meets with no great
difficulty on the grounds that the nuclei are distinct centres of
metabolic activity, for we know at present so little of what sort of
action takes place between the nucleus and the protoplasm that we cannot
rest our argument on any demonstrable relation.
The discovery that pieces below a certain minimum size are incapable of
producing a whole organism is of capital importance. It has been pointed
out that pieces of the egg of the sea-urchin less than one-sixteenth of
the whole do not produce even the gastrula stage. In amphioxus the
one-eighth blastomere seems to be near the lower limit of development.
It has also been found that there is a lower limit for pieces of adult
organisms below which they do not regenerate. This has been shown for
hydra, tubularia, planarians, and stentor, and is probably true for all
forms. This result is especially interesting in those cases in which the
parts contain all the elements necessary to produce a new organism, and
come from parts of the body that are totipotent in these respects. It
seems certain that the lack of power of development in these cases is
due entirely to the smallness of the piece. We can express the idea in
another way by stating that a certain volume is necessary in order that
a piece may produce the typical organization. This conclusion is
important as showing that the organization is something enormously large
as compared with the size of the chemical or physical molecules, and
even of the crystal molecule. The size of a piece that is at the lower
limit of organization is also very much larger than the smallest cells
of which the embryo is made up, and this relation is a point in favor of
the view that the organization is not simply the resultant of the
interaction of the cells, but is something much larger than these cells;
and we may even go further, I think, and add that it dominates the cells
rather than is controlled by them.
In the light of the questions discussed in the preceding pages, we may
now attempt to follow out in a more connected way some of the modern
views and hypotheses dealing with the problem of development.
Hertwig, as we have seen, has opposed the Roux-Weismann hypothesis, and
has formulated a view of his own. According to Hertwig, the cleavage
divides the egg into equivalent parts,--an idea very similar to that of
Pflüger. The cells he regards as units, and the development as the
result of the interaction of the cells,--a process that in a way Roux
had also assumed to take place between the different parts of the later
embryo. Thus, while Hertwig’s hypothesis contains little that is really
new, it has selected portions from several already existing hypotheses,
and united them into a consistent whole. It has been objected to
Hertwig’s view that the interaction of equivalent cells could never
account for the introduction of new processes in the development; but if
we grant that the cells are never entirely equivalent, whatever their
potence may be, this objection can, I think, be met. Hertwig’s chief
service has been his destructive criticism of the Roux-Weismann idea of
qualitative nuclear division.
Hertwig maintains that each stage in the development is the cause of the
next stage, and states that a description of the series of stages
through which the embryo passes gives a causal knowledge of the
phenomena of development. He claims that beyond this descriptive
knowledge we cannot hope to penetrate. Both Roux and Driesch have taken
issue with Hertwig, and have pointed out that while each stage in the
development contains within itself the causes of the succeeding stage,
yet we gain no idea as to these causes from a simple description of two
consecutive stages themselves. To state that the fertilized egg is the
cause of the cleavage gives us no idea of what sort of a process the
cleavage is, or how it arises, or what determines the sequence of the
divisions, etc. The blastula, for instance, contains the factors that
produce the gastrula; but to state that, in a physical sense, the
blastula is the cause of the gastrula is an erroneous interpretation of
what is meant by causal knowledge. If Hertwig’s idea were correct, there
would be as many causes in each embryo as there are stages in its
development, and as many causes in the whole range of embryology as
there are forms that develop multiplied by the number of stages in each
embryo. What we should seek to discover is the particular cause that
brings about each kind of process. If we could discover the cause in one
single case, it is highly probable that it would be found to extend to a
large number of other cases.
Driesch formulated an hypothesis of development in his _Analytische
Theorie_, but has modified and changed it in several later
contributions. It is difficult to give in a few words the subtile
analysis which Driesch has made of the phenomena of development. His
analytical theory rests on the dictum that the prospective value of each
blastomere is a function of its position in the whole. By “function” is
meant “a relation of dependence of a general unknown kind.” This idea is
connected with the following one, viz. that any blastomere could be
interchanged with any other one without altering the end-result. A few
elementary processes are supposed to be “given” in the structure, or in
the composition of the egg. Each elementary process is the outcome of a
cause, and each elementary process must release the succeeding
causes,--_i.e._ if the organization of the phase A is present, one of
the causes of the next phase B is also then present. The first
elementary process is the cleavage, that is initiated (“ausgelöst”) by
the fertilization. After a fixed number of divisions has taken place,
the cleavage process comes to an end. It has led to the production of a
number of cells having similar nuclei but having a different plasma
structure, and the result is the blastula stage. Organs whose formation
starts from the blastula stage are called primary organs; the
archenteron, the mesenchyme, the ciliated band, and the mouth of the
sea-urchin embryo belong to this class. Secondary organs are those that
arise from the primary ones, as the cœlom sacs, for instance, in the
sea-urchin embryo. The primary organs are started by the setting free
(“Auslösung”) of a new elementary process in the blastula, and later the
secondary organs are started by new elementary processes that arise in
the gastrula, which cannot appear until the gastrula stage itself is
present as a starting-point. In other words, the elementary processes
that are “given” in the egg can only come into action, or be set free
after a certain stage has come into existence. This means that we must
think of each organ that responds to a stimulus as having the
possibility of receiving that stimulus, and also of answering to it.
Even in inorganic nature every reaction must depend on a specific
receptiveness and a specific answer. Driesch supposes that the
receptivity is in the protoplasm, and the power to respond is in the
nucleus of each cell. In this way he attempts to meet the difficulty
that the nucleus is, in every cell, the bearer of the totality of all
the “Anlagen”; but inasmuch as it is surrounded by a specific plasma, it
is in a position to receive only certain stimuli, and can therefore only
respond to certain causes.
In the specific nature of the cytoplasm of the cell lies the prospective
potence of every organ, and the possibilities of each cell are limited
by its plasma; the cell becomes more and more limited as development
proceeds. It may be said, therefore, that in the course of development
the cells become actually limited in their possibilities, although they
may still retain within themselves, in the nucleus, the potentialities
of the entire organism.
In the course of development each causal reaction brings about not only
chemically specific differences, and thereby makes possible the
introduction of new elementary processes, but the reaction also brings
about by this very means a lessening of the possibilities of the cell,
because each cell will now only respond to a more limited set of causes.
We may say that the elementary process is not only the cause of the next
change, but by virtue of its specific nature it is the beginning stage
of the future reactions. Development proceeds from a few prearranged
conditions, that are given in the structure of the egg, and these
conditions, by reacting on each other, produce new conditions, and these
may in turn react on the first ones, etc. With every effect there is at
the same time a new cause, and the possibility of a new specific action,
_i.e._ the development of a specific receiving station for stimuli. In
this way there develops from the simple conditions existing in the egg
the complicated form of the embryo.
In this brief summary of some of the essential features of Driesch’s
hypothesis, I have omitted some parts that seem to me to go beyond the
legitimate field of a scientific hypothesis,--such, for instance, as the
causal harmony of the reactions; and other parts have been omitted
because they are improbable in the light of more recent work. It would
not be difficult to show that many difficulties beset each stage of the
argument, or to show how slender a basis of fact there is to support
some of the hypotheses. In fact, Driesch himself has modified very
greatly some of the views of his _Analytische Theorie_ in his later
writings. The merits of the analysis should not be overlooked, however,
since it is one of the first philosophical attempts to show how, in the
light of recent discoveries, the process of epigenetic development may
receive a causal interpretation. Even if the argument should break down,
the hypothesis will remain an interesting contribution, opening the way
to newer points of view in regard to the process of development. In
later papers, especially in those dealing with the localization of
morphogenetic processes, Driesch attempts to show that certain
experimental results _demonstrate_ that there is a vitalistic principle
at work in the development of the organism from the egg, as well as in
the process of regeneration. He bases his argument on the results of the
experiment in which the gastrula of the sea-urchin egg is cut in two, as
described already on page 234. The archenteron has not, at the time of
the experiment, subdivided itself into its three characteristic parts.
The posterior piece, that contains the posterior part of the archenteron
(the anterior part having been removed with the anterior piece),
produces a new whole embryo of smaller size, in which the archenteron is
subdivided into three parts, that are in the same proportion to each
other and to the whole embryo as are the same divisions of the normal
archenteron. This proportionate formation of the parts of the
archenteron on a smaller scale cannot, Driesch claims, be accounted for
on any known chemical or physical principle. There must be, therefore, a
different sort of principle involved, and this Driesch calls the
vitalistic principle.
It may be pointed out that this illustration that Driesch has selected
is only an example of all proportionate development, which many
observers have described as taking place in pieces of embryos. It is
only a striking case of what has been also known in many cases of
regeneration, of small pieces producing whole structures, and there is
nothing new or startling in this demonstration of a vitalistic
principle. The fact may be stated in another way, viz. that the
proportionate development of an organ is, within certain limits,
self-determining, or is self-determined by its size. The vitalistic
principle that Driesch sees demonstrated in these results is the now
familiar process of a smaller piece producing the typical structure on a
smaller scale; a phenomenon that a number of other writers had already
called attention to as one of the most remarkable phenomena connected
with the regeneration of pieces of an adult organism, or of an egg.
It is something of this same sort that the older zoologists must have
had in mind when they spoke of “formative forces” as peculiar to living
things. The use of the word “force” in this connection has often been
objected to, and not without justification; since it seems to imply that
the action is of the sort for which the physicist uses the word “force.”
The fundamental question turns upon whether the development of a
specific form is the outcome of one or more “forces,” or whether it is a
phenomenon belonging to an entirely different category from anything
known to the chemist and the physicist. If we state that it is the
property of each kind of living substance to assume under certain
conditions a more or less constant specific form, we only restate the
result without referring the process to any better-known group of
phenomena. If we attempt to go beyond this, and speculate as to the
principles involved, we have very little to guide us. We can, however,
state with some assurance that at present we cannot see how any known
principles of chemistry or of physics can explain the development of a
definite form by the organism or by a piece of the organism. Indeed, we
may even go farther and claim that it appears to be a phenomenon
entirely beyond the scope of legitimate explanation, just as are many
physical and chemical phenomena themselves, even those of the simplest
sort. To call this a vitalistic principle is, I think, misleading. We
can do nothing more than claim to have discovered something that is
present in living things which we cannot explain and perhaps cannot even
hope to explain by known physical laws.
Wilson (’94) has also rejected Roux’s hypothesis of qualitative nuclear
division, and adopts the view of the totipotence of the early
blastomeres. He has also advanced the view that there is during
development a progressive differentiation of the cells. In a later
contribution (’96), he accepts “the view of Hertwig and of Driesch that
the various degrees of partial development beginning with the echinoderm
egg and culminating in the gasteropod may be due to varying conditions
of the egg cytoplasm in the different forms.” Wilson points out that the
series of forms represented at one end by amphioxus and at the other end
by the ctenophore and the gasteropod may be brought under a common point
of view, “for it is certain that development must be fundamentally of
the same nature throughout the series, and the differences must be of
secondary moment.”
If we reject, as several students of experimental embryology and of
regeneration have done, the Roux-Weismann idea of the existence of
pre-formed germs in the nucleus, and also the idea of Hertwig of the
equivalency of the first-formed blastomeres, and Driesch’s vitalistic
principle, what position can we take in regard to the problem of
development? We may at least attempt to formulate our present position.
There must be assumed to exist in the egg an organization of such a kind
that it can be divided and subdivided during the cleavage without
thereby losing its primary character. The refusion of the cells after
each division by means of protoplasmic connections indicates how this
may be possible. The organization must be thought to be of such a kind
that the factors determining the cleavage may be different from those
that determine the median plane of the body. This is demonstrated by
Pflüger’s experiment in which the position of the cleavage planes is
changed, but the embryo appears in relation to the primary meridians.
The first-formed blastomeres, that result from the division of the egg,
do not seem to be strictly equivalent, but they appear to be in most
cases, at least, totipotent. The characteristics of each part of the
protoplasm may be a factor in determining what sort of structure may
come from that part of the egg, but back of this lies the fundamental
character of the protoplasm itself, that determines what each part, in
its relation to the whole, can do. The division of the nucleus appears
to be in all cases an exact quantitative division, and there is some
evidence to show that the early nuclei are all equivalent,--or at least
totipotent. The division of the protoplasm is often into unlike parts,
and the kind of cytoplasm contained in a part may or may not limit the
potencies of each part.
One of the most important facts in connection with the organization is
that a part, if separated from the rest, may become a new whole, and
this appears to be a fundamental peculiarity of living things.
Analogies can be found, perhaps, in inorganic phenomena, as for instance
a storm dividing into two or more parts and each developing a new storm
centre of its own, or when a suspended drop is divided and each half
becomes a new sphere; but these comparisons lack some of the essential
features of the organic phenomenon.
A progressive change takes place as development proceeds, so that a
stage once passed through is not repeated if a part is separated from
the rest, as illustrated by Driesch’s experiments with the blastula and
gastrula of the sea-urchin and starfish, and by the method of
development of pieces of the adult, that do not pass through the
embryonic stages. As the protoplasm changes new conditions may arise,
either because the protoplasm in its new form can be acted upon by those
internal or external conditions to which it did not respond at first, as
Driesch has supposed, or, as I think equally probable, because the
series of reactions that have begun with the first step in the
development work themselves out in the same way that a chemical reaction
once started may pass through a long series of stages depending upon the
nature of the substance. The difference between these views lies in
this, that the former supposes latent substances, or elementary
processes or forces, whatever they may be called, to be present in the
egg and to act when a medium that responds to them has come into
existence; the other idea supposes that the whole process is started
with the first change and once set going is of such a kind as to
continue to an end through a regular series of stages. Both views are
suppositions, and, it may be, reduce themselves ultimately to the same
thing.
On any theory of development, the nucleus cannot be left out of account,
since the evidence that we now possess shows that through the nucleus
even the most trivial peculiarity of one parent, and probably of both,
may be transmitted. This has led a number of zoologists to look upon the
nucleus as a body containing specific elements corresponding to those of
the individual from which the nucleus has come, but inheritance through
the nucleus is no more a demonstration of the existence of pre-formed
elements of the male than are the general facts of embryology a
demonstration of pre-formation. All we can legitimately conclude is that
the substance of the nucleus is of such a sort that it acts on the
cytoplasm in a definite way, and determines, in part at least, its
differentiation. There has been steadily accumulating evidence to show
that during development there is an interchange of material between the
nucleus and the protoplasm, and it is not going far afield to conclude
that the character of both nucleus and protoplasm is altered by the
interchange in material. If this is admitted it is no more remarkable
that a hybrid is midway between its parents than that a parthenogenetic
egg produces a form like that of the individual from which it has come.
Several writers, as we have seen, have adopted the view that the nuclei
are storehouses of the undifferentiated germ plasm, and retain
everywhere the sum total of the “Anlagen” of the egg nucleus. I do not
know of any evidence that demonstrates that the nucleus is less modified
in these regards than is the rest of the cell. On the contrary it seems
to me that a fair case might be established in favor of the view that
the nucleus and the cytoplasm cannot be contrasted in this way, and that
a change in the cytoplasm may also involve a change in the nucleus.
The phenomena of regeneration show over and over again that
differentiated cells may change into structures entirely different from
what they have been, as illustrated in the development of the lens from
the edge of the iris, and in the production of a new hydra, or
tubularian, from a piece of an old one. It is, I think, an arbitrary
assumption to suppose that this is brought about by a reserve stuff in
the nucleus, for the production of new eggs and spermatozoa in the
animal, from cells that have themselves passed through most of the early
embryonic changes and have been parts of embryonic organs, shows that
although the protoplasm may change throughout these stages, it may still
come back to the starting-point, and there is nothing to show that this
return is brought about by the nucleus. I cannot but think that Driesch
was prejudiced by current opinion, when he adopted the view, as one of
the foundations of his analytical theory, that the nucleus contains all
the “Anlagen” of the whole organism, and that the protoplasm alone
undergoes a progressive change.
The central problem for embryology is the determination of what is the
cause or causes of differentiation. Our analysis leads us to answer that
it is the outcome of the organization; but what is the organization?
This it must be admitted is a question that we cannot answer. Looked at
in this way the problem of development seems an insoluble riddle; but
this may be because we have asked a question that we have no right to
expect to be answered. If the physicist were asked what is gravity he
could give no answer, but nevertheless one of the greatest discoveries
of physics is the law of gravitation. If we could answer the question of
what the organization is to which we attribute the fundamental
phenomenon of development, there would perhaps be nothing further left
to find out in the development of animals. Fortunately there is a
different and safer point of view. There are other questions to which we
can expect an answer. Because the physicist cannot tell what gravity is,
he neither rejects the term nor despairs of obtaining a knowledge of how
it acts. If our analysis of the problem of development leads us to the
idea of an organization existing in the egg, our next problem is to
discover how it acts during development. Most of the results described
in several of the preceding chapters have taught us something of how
the organization behaves. We have found that it can be affected by
external circumstances, even to such an extent that its polarity may be
reversed. We have seen that if an organized structure is broken up into
pieces, each piece may reorganize itself into a new whole. The most
familiar, and at the same time the most difficult thing to understand,
is that the organization is of such a kind that it has the property of
passing through a definite series of stages leading to a typical result,
and having reached its goal of throwing off organized bodies, or germ
cells, that begin once more at the starting-point and pass through the
same cycle. The action of the organism is sometimes compared to that of
a machine, but we do not know of any machine that has the property of
reproducing itself by means of parts thrown off from itself.
These are some of the most characteristic phenomena exhibited by the
organization. In the final chapter some of the questions that have been
suggested in connection with the method of action of the organization
will be further discussed.