KARL ERNST VON BAER
Von Baer was recognised as the founder of embryology even by his
contemporaries. His predecessors, Aristotle,[166] Fabricius,[167]
Harvey,[168] Malpighi,[169] Haller,[170] Wolff,[171] had made a
beginning with the study of development; von Baer, by the thoroughness
of his observation and the strength of his analysis, made embryology a
science.
It was to one of the German transcendentalists that von Baer owed the
impulse to study development. Ignatius Doellinger, Professor in Wuerzburg,
induced three of his pupils, Pander, d'Alton and von Baer, to devote
themselves to embryological research. The development of animals was at
this time little known, in spite of recent work by Meckel (1815 and
1817), Tiedemann (_Anatomie u. Bildungsgeschichte des Gehirns_, 1816),
by Oken (_loc. cit., supra_, p. 90), and some others.
Pander, with whom apparently Doellinger and d'Alton collaborated, was the
first to publish his results;[172] von Baer, who through absence from
Wuerzburg had for a time dropped his embryological studies, started to
work in 1819, after the publication of Pander's treatise, and produced
in 1828 the first volume of his master-work, _Ueber
Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion_
(Koenigsberg, 1828). The second volume followed in 1837, but dates really
from 1834, and was published in an incomplete form. This second volume
is intended as an introduction to embryology for the use of doctors and
science students. In it von Baer describes in full detail the
development of many vertebrate types--chick, tortoise, snake, lizard,
frog, fish, several mammals and man, basing his remarks largely upon his
personal observations, but taking account also of all contemporary work.
A separate account of the development of a fish (_Cyprinus blicca_)
appeared in 1835.[173]
We shall concentrate attention on the first volume. This volume contains
the first full and adequate account of the development of the chick,
followed by a masterly discussion of the laws of development in general.
When we consider that von Baer worked chiefly with a simple microscope
and dissecting needles, the minuteness and accuracy of his observations
are astonishing. He described the main facts respecting the development
of all the principal organs, and if, through lack of the proper means of
observation, he erred in detail, he made up for it by his masterly
understanding and profound analysis of the essential nature of
development. His account of the development of the chick is a model of
what a scientific memoir ought to be; the series of "Scholia" which
follow contain the deductions he made from the data, and, in so far as
they are direct generalisations from experience, they are valid for all
time.
The first Scholion is directed against the theory of preformation, and
succeeds in refuting it on the ground of simple observation. The theme
of the second Scholion is that the essential nature (_die Wesenheit_) of
the animal determines its differentiation, that no stage of development
is solely determined by the antecedent stage, but that throughout all
stages the _Wesenheit_ or idea of the definitive whole exercises
guidance. This guidance is shown most clearly in the regulatory
processes of the germ, whereby the large individual variations commonly
presented by the early embryo are compensated for or neutralised in the
course of further development. Baer in this shows himself a vitalist.
It is, however, the third and subsequent Scholia which must here
particularly occupy our attention, for it is in these that von Baer
comes to grips with morphological problems. Already in the second
Scholion he had definitely enunciated the law which runs as a theme
throughout the volume, the observational and the theoretical part alike,
the law that development is essentially a process of differentiation by
which the germ becomes ever more and more individualised. "The essential
result of development," he writes, "when we consider it as a whole, is
the increasing independence (_Selbstaendigkeit_) of the developing
animal" (p. 148). In the third Scholion he elaborates this thought and
shows that differentiation takes place in triple wise. The three
processes of differentiation are "primary differentiation" or
layer-formation, "histological differentiation" within the layers, and
the "morphological differentiation" of primitive organs.
The first of these differentiations in time is the formation of the
germ-layers, which takes place by a splitting or separation of the
blastoderm into a series of superimposed lamellae. Baer's account of the
process in the chick is as follows:--
"First of all, the germ separates out into heterogeneous layers, which
with advancing development acquire ever greater individuality, but even
on their first appearance show rudiments of the structures which will
characterise them later. Thus in the germ of the bird, so soon as it
acquires consistency at the beginning of incubation, we can distinguish
an upper smooth continuous surface and a lower more granular surface.
The blastoderm separates thereupon into two distinct layers, of which
the lower develops into the plastic body-parts of the embryo, the upper
into the animal parts; the lower shows clearly a further division into
two closely connected subsidiary layers--the mucous layer and the
vessel-layer; the original upper layer also shows a division into two,
which form respectively the skin and the parts which I have called the
true ventral and dorsal plates--parts which contain in an
undifferentiated state the skeletal and muscular systems, the connective
tissues, and the nerves belonging to these. In order to have a
convenient term for future use, I have named this layer the
muscle-layer" (p. 153).
The process of delamination results then in the formation of four
layers, of which the upper two (composing the "animal" or "serous"
layer) will give origin to the animal (neuromuscular) part of the body,
the lower pair to the plastic or vegetative organs. The uppermost layer
will form the external covering of the embryo, and also the amniotic
folds; from it there differentiates out at a very early stage the
rudiment of the central nervous system, forming a more or less
independent layer. Below the outermost layer lies the layer from which
are formed the muscular and skeletal systems, and beneath this
"muscle-layer" comes the "vessel-layer," which gives origin to the main
blood-vessels. The innermost layer of the four will form the mucous
membrane of the alimentary canal and its dependencies; at the present
stage, however, it is, like the other layers, a flat plate.
From all these layers tubes are developed by the simple bending round of
their edges. The outermost layer becomes the investing skin-tube of the
embryo; the layer for the nervous system forms the tubular rudiment of
the brain and spinal cord; the mucous layer curls round to form the
alimentary tube; the muscle layer grows upwards and downwards to form
the fleshy and osseous tube of the body wall; even the vessel layer
forms a tube investing the alimentary canal, but a part of it goes to
form the medial "Gekroese," or mesenterial complex, which departs
considerably from the tubular form.
When these tubes or "fundamental organs" are formed the process of
primary differentiation is complete. The fundamental organs, however,
have at no time actually the form of tubes; they exist as tubes only
ideally, for morphological and histological differentiation go on
concurrently with the process of primary differentiation.
Through morphological differentiation the various parts of the
fundamental organs become specialised, through unequal growth, first
into the primitive organs and then into the functional organs of the
body. "Single sections of the tubes originally formed from the layers
develop individual forms, which later acquire special functions: these
functions are in the most general way subordinate elements of the
function of the whole tube, but yet differ from the functions of other
sections. Thus the nerve-tube differentiates into sense-organs, brain
and spinal cord, the alimentary tube into mouth cavity, oesophagus,
stomach, intestine, respiratory apparatus, liver, bladder, etc. This
specialisation in development is bound up with increased or diminished
growth" (p. 155). Rapid growth concentrated at one point brings about an
evagination; in this manner are formed the sense-organs from the
nerve-tube, the liver and lungs from the alimentary tube. Or increased
growth over a section of a tube causes it to swell out; in this wise the
brain develops from the nerve-tube, the stomach from the alimentary
tube. The segmentation which soon becomes so marked, particularly in the
muscle layer, is also due to a process of morphological differentiation.
At the same time that the organs of the body are being thus roughly
blocked out and moulded from the germ-layers the third process of
differentiation is actively going on. "In addition to the
differentiation of the layers, there follows later another
differentiation in the substance of the layers, whereby cartilage,
muscle and nerve separate out, a part also of the mass becoming fluid
and entering the bloodstream" (p. 154). Through histological
differentiation the texture of the layers and incipient organs becomes
individualised. In its earliest appearance the germ consists of an
almost homogeneous mass, containing clear or dark globules suspended in
its substance (ii., p. 92). This homogeneity gives place to
heterogeneity; the structureless mass becomes fibrous to form muscles,
hardens to form cartilage or bone, becomes liquid to form the blood,
differentiates in a hundred other ways--into absorbing and secreting
tissues, into nerves and ganglia, and so forth. It will be noticed that
the concept of histological differentiation is independent of the
cell-theory; it signifies that textural differentiation which leads to
the formation of tissues in Bichat's sense. The tissues and the
germ-layers stand in fairly close relation with one another, for while
certain tissues are formed chiefly but not exclusively in one layer,
others are formed only in one layer and never elsewhere. For example,
peripheral nerves are for the most part formed in the muscle layer,
though the bulk of the nervous tissue is formed in the walls of the
nerve tube; similarly blood and blood-vessels may arise from almost any
layer, though their chief seat of origin is the vessel-layer; on the
other hand, bone is formed only in the muscle-layer (i., p. 155, ii.,
pp. 92-3).
This relation of tissue to germ-layer was more fully discussed and
brought into greater prominence by Remak, from the standpoint of the
cell-theory, and it will occupy us in a later chapter (Chap. XII.).
The fourth Scholion elaborates the analysis of developmental processes
still further, and discusses in particular the scheme of development
which is shown by the Vertebrata. The characteristic structure of the
vertebrate body is brought about by a "double symmetrical" rolling
together of the germ-layers, whereby two main tubes are formed, one
above and one below the axis of the body, which is the chorda. The
dorsal tube is formed by the two animal layers, the ventral tube by all
the layers combined (see Fig. 7).
The process is indicated with sufficient clearness in the diagram. It
will be seen that the real foundation and framework of the arrangement
is the muscle-layer, with its two tubes, one surrounding the central
nervous system and forming the "dorsal plates," the other surrounding
the body cavity and forming the "ventral plates." In the dorsal plates,
which early show metameric segmentation, the investing skeleton of the
neural axis develops; in the ventral plates are formed the ribs, the
ventral arches of the vertebrae, the hyoid, the lower jaw and other
skeletal structures.
The alimentary or "mucous" tube and the part of the vessel layer which
invests it become so closely bound up with one another as to form a
single primitive organ--the alimentary canal. The muscles of the
alimentary canal are accordingly in all probability developed in the
investing part of the vessel layer. From the "Gekroese," or remaining
part of the vessel layer develop the Wolffian bodies (_Urnieren_,
Pronephros), the kidneys, the sex glands, and the series of
"blood-glands"--suprarenals, thyroid, thymus and spleen. Baer did not
attach any special morphological significance to the peritoneal lining
of the body cavity, as is done in more modern forms of the germ-layer
theory. The gill-slits were largely formed by outgrowths from the
alimentary canal.
_a._ Chorda.
_b._ Dorsal plates.
_c._ Ventral plates.
_d._ Spinal cord.
_e._ Vessel-layer.
_f._ Alimentary tube.
_g._ Pronephros.
_h._ Skin.
_i._ Amnion.
_k._ Serous membrane.
_l._ Yolk-sac.
In his germ-layer theory von Baer was influenced a good deal by
Pander, to whom the actual discovery of the process of layer-formation
is due. Pander, however, had distinguished only three germ-layers, an
upper "serous" layer, a lower "mucous" layer and a middle
"vessel-layer." He it was who introduced the terms "Keimhaut"
(blastoderm) and "Keimblatt" (germ-layer).
[Illustration: FIG. 7.--Ideal Transverse Section of a Vertebrate Embryo.
(After von Baer.)]
The honour of being the founder of the germ-layer theory is sometimes
attributed to C. F. Wolff, notably by Koelliker and O. Hertwig. Wolff, it
is true, in his memoir _De formatione intestinorum_ (1768-9) showed that
the alimentary canal was first formed as a flat plate which folded round
to form a tube, and in a somewhat vaguely worded passage he hinted that
a similar mode of origin might be found to hold good for the other
organ-systems. But it seems clear that Wolff had no definite conception
of the process of layer-formation as the first and necessary step in all
differentiation. This, at any rate, was von Baer's opinion, who assigns
to Pander the glory of the discovery of the germ-layers. "You," he
writes, "through your clearer recognition of the splitting of the
germ--a process which remained dark to Wolff--have shed a light upon all
forms of development" (p. xxi.).
We have now seen, following von Baer's exposition, how development is
essentially a process of differentiation, a progress from the general to
the special, from the homogeneous to the heterogeneous; we have analysed
the process into its three subordinate processes--primary, histological
and morphological differentiation. So far we have considered development
in general and the laws which govern it; we have now to consider the
varieties of development which the animal kingdom offers in such
profusion, in order to discover what relations exist between them. This
is the problem set in the fifth Scholion. Baer at once brings us face to
face with the solution of the problem attempted in the Meckel-Serres
law. It is a generally received opinion, he writes, that the higher
animals repeat in their development the adult stages of the lower, and
this is held to be the essential law governing the relation of the
variety of development to the variety of adult form. This opinion arose
when there was little real knowledge of embryology; it threw light
indeed upon certain cases of monstrous development, but it was pushed
altogether too far. It complicated itself with a belief in a historical
evolution;--"People gradually learnt to think of the different animal
forms as developed one from another--and seemed, in some circles at
least, determined to forget that this metamorphosis could only be
conceptual" (p. 200). At the same time the theory of parallelism led men
to rehabilitate the outworn conception of the scale of beings, to
maintain that animals form one single series of increasing complexity, a
scale which the higher members must mount step by step in their
development--from which it followed that evolution, whether conceived as
an ideal or as an historical process, could take place only along one
line, could be only progressive or regressive. Not all the supporters of
the theory of parallelism held these extreme views, but conclusions of
this kind were natural and logical enough.
Von Baer had soon found in the course of his embryological studies that
the facts did not at all fit in with the doctrine of parallelism; the
developing chick, for example, was at a very early stage demonstrably a
Vertebrate, and did not recapitulate in its early stages the
organisation of a polyp, a worm or a mollusc. He had published his
doubts in 1823, but his final confutation of the theory of parallelism
is found in this Scholion.
If it were true, he says, that the essential thing in the development of
an animal is this repetition of lower organisations, then certain
deductions could be drawn, which one would expect to find confirmed in
Nature. The first deduction would be that no structures should appear in
the embryo of the higher animals that are not found in the lower
animals. But this is not confirmed by the facts--no adult among the
lower animals, for instance, has a yolk-sac like that of the chick
embryo. Again, if the law of parallelism were true, the mammalian embryo
would have to repeat the organisation of, among other groups, insects
and birds. But the embryo _in utero_ is surrounded by fluid and cannot
possibly breathe free air, so it cannot possibly repeat the structure of
either insects or birds, which are pre-eminently air-organisms.
Generally speaking, indeed, we find in all the higher embryos special
structures which adapt them to the very special conditions of their
development, and these we never find as permanent structures in the
lower animals. The supporters of the theory of parallelism might,
however, admit the existence of such special embryonic organs without
greatly prejudicing their case, for these temporary organs stand to some
extent outside the scope of the theory.
But they would have to face a second and more important deduction from
their views, namely, that the higher animals should repeat at every
stage of their development the whole organisation of some lower animal,
and not merely agree with them in isolated details of structure. The
deduction is, however, not borne out by the facts. The embryo of a
mammal resembles in many points, at different stages of its development,
the adult state of a fish; it has gill-slits and complete aortic arches,
a two-chambered heart, and so on. But at no time does it combine all the
essential characters of a fish; nor has it ever the tail of a fish, nor
the fins, nor the shape. Any recapitulation there may be is a
recapitulation of single organs, there is never a repetition of the
complete organisation of a fish. This is indeed the fundamental
criticism of the theory of parallelism; and if it applies even within
the limits of the vertebrate phylum, so much the more does it apply to
comparisons between embryonic Vertebrates and adult Invertebrates.
There are also some lesser arguments which might be urged against the
theory of parallelism. If the theory were strictly true, no state which
is permanent in a higher animal could be passed through by an animal
lower in the scale. But birds, which are lower in the scale than
mammals, pass through a stage in which they resemble mammals in certain
respects much more than they do when adult, for in an embryonic
condition they agree with mammals in having no feathers, no air sacs, no
pneumatic sacs in the bones, no beak. Their brain also resembles that of
mammals more in an earlier stage than it does later. So, too, myriapods
and hydrachnids have at birth three pairs of feet, and resemble at this
stage adult insects, which form a higher class.
Again, were the analogy between the development of the individual and
the evolution of the _Echelle des etres_ complete, organs and
organ-systems ought to develop in the individual in the order in which
they appear in the scale of beings. But this is not always the case. In
fish the hinder extremity develops only its terminal joint, while in the
embryos of higher animals the basal joint is the first to appear.
Another consequence one would expect to find realised, were the theory
of parallelism correct, is the late appearance in development of parts
which are confined to the higher animals. In the development of a
Vertebrate accordingly one would not expect the vertebrae to appear
before the embryo had passed through many Invertebrate stages. But
experience shows the direct contrary, for in the chick the rudiments of
the vertebral axis appear sooner than any other part.
The theory of parallelism or recapitulation then is not borne out by the
facts, and clearly cannot be the law which we are seeking. But what then
is the true relation between the variety of development and the variety
of adult structure? Before answering this question we must review the
varied forms of adult organisation and consider in what relations they
stand to one another. In particular we must enquire whether they belong
to one type or to many. One point is here cardinal--we must distinguish
between the _type_ of organisation and the _grade_ of differentiation.
By "type" von Baer means the structural plan of the organism. "I call
the _type_ the spatial relationship of the organic elements and organs"
(p. 208). Each type of organisation characterises one of the big groups
of animals; the lesser groups represent "grade" modifications of the
type. "The product of the degree of differentiation and the type gives
the several great groups of animals which are called classes" (p. 208).
_Ausbildung_ (differentiation) takes place in one or other of several
directions, in adaptation, for instance, to life in the water or to life
in the air.
There are, von Baer considers, four main types--(1) the peripheral or
radiate type, (2) the longitudinal type, (3) the massive or molluscan
type, (4) the vertebrate type. The radiate type is shown by discoid
infusoria, by medusae, by starfish and their allies. The longitudinal
type characterises such genera as _Vibrio_, _Filaria_, _Gordius_, and
all the annulate animals. Mollusca, rotifers, polyzoa, and such
infusoria as are not included in types (1) and (2) belong to the massive
type, in which the body and its parts form rounded masses. The
longitudinal type is predominantly "animal," the massive type
predominantly "plastic" (vegetative). The vertebrate type has both the
"animal" and the "plastic" organs highly developed. In the symmetrical
arrangement of the animal parts it resembles the longitudinal type; its
plastic parts with their asymmetrical arrangement and rounded shape
belong to the massive type.
These types of von Baer inevitably recall the "Embranchements" of
Cuvier, with which they more or less coincide. It seems that von Baer
arrived at his types (from the study of adult structure) independently
of Cuvier, though the priority of publication rests with Cuvier.[174]
Now it is clear that the development of the individual, which is
essentially an _Ausbildung_, a differentiation, is directly comparable
with the grade-differentiation of forms within the type. And just as the
type rules all its varied modifications, so does the development of the
individual take place always within the bounds imposed by type. This is
von Baer's chief contribution to the theory of embryonic
relationships--the law that "the type of organisation determines the
manner of development" (p. xxii.). Development is not merely from the
general to the special--there are at least four distinct "general"
types, from which the special is developed. The type is fixed in the
very earliest stages of development--the embryo of a Vertebrate is from
the very beginning a Vertebrate (p. 220), and it shows at no time any
agreement in total organisation with any Invertebrate. The types are
independent of one another; differentiation and development follow a
different course in each of them. Not but what some analogies can be
found between the very earliest stages of embryos of different type.
Thus vertebrate and annulate embryos agree in certain points at the time
of the formation of the primitive streak. And in the earliest stage of
all, the egg-stage, there is probably agreement between all the types.
In eggs with yolk, whether vertebrate or annulate, there is always a
separation into an animal and a plastic layer. It seems, too, as if a
hollow sphere were a constant stage in the development of all animals
(pp. 224, 258). Apart from these analogies, development takes an
entirely independent course in each of the four main types, and no
embryo of one of the higher types repeats in its development the
peculiar organisation of any adult of the lower types.
If we consider now development within the type, which is the only
legitimate thing to do, we arrive at certain laws governing the relation
of embryos to one another. For instance, at a certain stage vertebrate
embryos are uncommonly alike. Von Baer had two in spirit which he was
unable to assign to their class among amniotes; they might have been
lizard, bird, or mammal, he could not say definitely which.[175] Generally
the farther back we go in the development of Vertebrates the more alike
we find the embryos. The type-characters are first to appear, then the
class characters, then the characters distinguishing the lesser
classificatory groups. "From a more general type the special gradually
emerges" (p. 221). The chick is first a Vertebrate, then a
land-vertebrate, then a bird, then a land-bird, then a gallinaceous
bird, and finally _Gallus domesticus_. Development within the type is a
progress from the general to the special, a real evolution. The more
divergent two adults are, the farther back we must go in their
development to find an agreement between their embryos. We can sum up
the case in the following laws:--
"(1) _That the general characters of the big group to which the embryo
belongs appear in development earlier than the special characters._ In
agreement with this is the fact that the vesicular form is the most
general form of all; for what is common in a greater degree to all
animals than the opposition of an internal and an external surface?
"(2) _The less general structural relations are formed after the more
general, and so on until the most special appear._
"(3) _The embryo of any given form, instead of passing through the state
of other definite forms, on the contrary separates itself from them._
"(4) _Fundamentally the embryo of a higher animal form never resembles
the adult of another animal form, but only its embryo_" (p. 224).
These laws relating to development within the limits of type are
destructive of even a limited application of the theory of parallelism,
for not even within the limits of the type is there a real scale which
the higher forms must mount; each embryo develops for itself, and
diverges sooner or later from the embryos of other species, the
divergence coming earlier the greater the difference between the adult
forms. It is only because the lower less-differentiated adult forms
happen to be little divergent from the generalised or embryonic type,
that they show a certain similarity with the embryos of the higher more
differentiated members of the group. Such similarity, however, is due to
no necessary law governing the development of the higher animals; it is,
on the contrary, merely a consequence of the organisation of these lower
animals (p. 224).
Von Baer goes on to show what are the distinguishing embryological
characters of the types and classes, working out a dichotomous schema of
development, which each embryo must follow, branching off early or late
to its terminal point, according to the lower or higher goal it has to
reach.
One important consequence for morphology results from von Baer's laws of
differentiation within the type. If the embryo develops from the general
to the special, then the state in which each organ or organ-system first
appears must represent the general or typical state of that organ within
the group. Embryology will therefore be of great assistance to
comparative anatomy, whose chief aim it is to discover the generalised
type, the common plan of structure, upon which the animals of each big
group are built. And the surest way to determine the true homologies of
parts will be to study their early development. "For since each organ
becomes what it is only through the manner of its development, its true
value can be recognised only from its method of formation. At present,
we form our judgments by an undefined intuition, instead of regarding
each organ merely as an isolated product of its fundamental organ, and
discerning from this standpoint the correspondences and dissimilarities
in the different types" (p. 233). Parts, therefore, which develop from
the same "fundamental organ," and in the last resort from the same
germ-layer, have a certain kinship, which may even reach the degree of
exact homology.
Now since the mode of development in each type is peculiar to that type,
organs of the same name in different types must not necessarily be
accounted homologous, even if they correspond exactly with one another
in their general _functional_ relations to the rest of the organs. Thus
the central nervous system of Arthropods must not be homologised with
the central nervous system of Vertebrates, for it develops in a
different manner. So, too, the brain of Arthropods or of Mollusca is not
strictly comparable with the brain of Vertebrates. Again, the air-tubes
or tracheae of insects are, like the trachea and bronchi of many
Vertebrates, air-breathing organs. But the two organs are not
homologous, for the air-tubes of Vertebrates are developed from the
alimentary tube ("fundamental organ" of the alimentary system, developed
from the vegetative layer), while the air-tubes of insects arise either
by histological differentiation, or by invagination of the skin (p.
236). Organs can be homologous only within the limits of the big groups;
there can be no question of homology between members of different types.
The development of plants, like the development of animals, is
essentially a progress from the general to the special (p. 242).
Botanists have not been troubled by any recapitulation theory, and in
founding their big groups, Acotyledons, Monocotyledons, and
Dicotyledons, upon embryological characters, they were guided by true
principles, which ought indeed to be followed in zoology. If we knew the
development of all kinds of animals sufficiently well, then the best way
to classify them would be according to the characters they show in their
early development, for it is in early development that they show the
characters of the type in their most generalised form. As it is, we have
in our ignorance to establish the big groups by the study of adult
structure, but we find, on putting together all we know of comparative
embryology, that a classification of animals according to the mode of
their development gives, as is only natural, the same four groups as
does the study of adult structure. The four types of development are
thus:--
(1) The double-symmetrical, which is found in Vertebrates. It is called
the double-symmetrical, because in Vertebrates development takes place
from a central axis (notochord) in two directions, upwards and
downwards, in such a way that two tubes are formed, one above and one
below the axis. (2) The second type is the symmetrical, which is shown
by Annulates. A primitive streak is formed on the ventral surface of the
yolk; development proceeds symmetrically on both sides of the streak.
(3) Radiate development is probably typical of the radiate structural
type. (4) In the massive type, the development seems to be a spiral one.
Common to most modes is a separation of the germ into animal and plastic
layers, a separation which seems to be conditioned largely by the
presence of yolk. A classification based upon embryological characters
ought to be applied even to the lesser groups and would here prove
itself of service. Embryology, for instance, fully supports de
Blainville's separation of Batrachia from true reptiles,[176] for reptiles
develop an amnion and Batrachia do not.
We come now to the sixth and last Scholion. Development is a true
evolution of the special from the general, so runs von Baer's most
general law of all. This can be expressed in a slightly different way,
and the words which he chooses in the sixth Scholion to express this
final and most general result are these:--"The developmental history of
the individual is the history of the growing individuality in every
respect" (p. 263). The greatest modern treatise on embryology ends on a
splendid note. One creative thought rules all the forms of life. And
more--"It is this same thought that in cosmic space gathered the
scattered masses into spheres and bound them together in the solar
system, the same that from the weathered dust on the surface of the
metallic planets brought forth the forms of life. And this thought is
nought else but life itself, and the words and syllables in which life
expresses itself are the varied forms of the living" (p. 264).
Von Baer reminds one greatly of Cuvier. There is the same sheer
intellectual power, the same sanity of mind, the same synthetic grip.
Von Baer, like Cuvier, never forgot that he was working with living
things; he was saturated, like Cuvier, with the sense of their
functional adaptedness. In his paper on the external and internal
skeleton[177] he gives a masterly analysis of the functional modifications
of the limbs in Vertebrates, and the whole paper indeed, with its sober
attack on transcendentalism, is a vindication as much of the functional
point of view as of the importance of embryology.
Both Cuvier and von Baer, by the very sanity of their views, found
themselves in partial opposition to the theories current in their time.
Cuvier was the critic of Geoffroy and the transcendentalists, of Lamarck
and the believers in the _Echelle des etres_, evolutionary or ideal. Von
Baer also, though influenced greatly by _Naturphilosophie_, turned
against the exaggerations of the transcendental school, and by his
unanswerable criticism of the theory of parallelism took away the ground
from those who too easily believed in an historical evolution.[178]
We have seen what were von Baer's criticisms of the theory of
parallelism. If we turn to the later writings of Cuvier we find the
essential criticism expressed in similar terms. Speaking of an attempt
which had been made to show that fish were molluscs developed to a
higher degree, he wrote in 1828,[179] "Let us draw the conclusion that
even if these animals can be spoken of as ennobled molluscs, as molluscs
raised to a higher power, or if they are embryos of reptiles, the
beginnings of reptiles, this can be true of them only in an abstract and
metaphysical sense, and that even this abstract statement would be very
far from giving an accurate idea of their organisation." From the fact
that the respiratory and circulatory organs of fish greatly resemble
those of tadpoles the conclusion has been drawn that fish are in a sense
embryos of Amphibia (p. 547). But this manner of viewing things is none
the less vicious, "for this reason ... that it considers only one or two
points and neglects all the others" (p. 548), and is directly contrary
to common sense. There is never a recapitulation of total organisations,
only at the most of single organs.
It will be remembered that Cuvier opposed and demolished the theory of
the _Echelle des etres_, not only by showing that there were in Nature
four entirely different plans of animal structure, but also by
demonstrating that even the animals of each single _Embranchement_ could
not readily be arranged in one series, that a serial arrangement was
really valid only for their separate organs. Von Baer also held that
there are four distinct types of structure; he, too, combated the idea
of gradation within the limits of the type. In so far as species
represent successive stages in the development, the _Ausbildung_, of the
type, so far can the idea of a scale of beings be applied. But the
members of a type follow not one line of evolution but several diverging
lines, in direct adaptation to different environmental conditions, so
that a serial arrangement of them is not as a rule possible. It may be
possible to establish a serial arrangement of single organs from the
simplest to the most complex. But each organ or organ-system will
require a different serial arrangement, for the different systems vary
on different lines and an animal may be highly developed in respect of
one system and little developed in respect of all the others. Man, for
instance, is the highest animal only in respect of his nervous system.
The idea of the scale of beings has therefore only a very limited
application even within the limits of the type. Applied to the whole
animal kingdom it becomes merely absurd.
Another point of resemblance between Cuvier and von Baer was that
Cuvier, though essentially a student of adult structure, did recognise
the importance of embryology; following up some observations of
Dutrochet he studied the foetal membrane of mammals and tried to
establish their homologies.[180] And in his criticism of the vertebral
theory of the skull he advanced as an argument against the
basisphenoid being a vertebral centrum the fact (established
by Kerkring, 1670), that it develops from two centres.[181] Von Baer's
relation to transcendental anatomy was in some ways a close one, though
he was a trenchant critic of the extreme views of the school.[182] He took
from Oken the idea that a simple fundamental plan rules the organisation
of all Vertebrates; "That jaws and limbs are modifications of one
fundamental form is readily apparent, and, after Oken, the fact ought to
be accepted by the majority of those naturalists who do not refuse to
admit the existence of a general type from which the diversity of
structure is developed" (i., p. 192). He accepted the vertebral theory
of the skull in its main lines, and used his embryological knowledge to
support the idea that jaws correspond to limbs--the latter point as part
of the transcendental idea that the hind end of the body repeats the
organisation of the anterior part (i., p. 192). The particular form
which his theory of the relation of jaws to limbs took is shown in the
following passage:--"The maxillary bone has ... the significance of an
extremity and at the same time that of a rib or lower arch of a
vertebra, just as the pelvic bones unite in themselves the signification
of ribs and proximal members of the hinder extremity" (Meckel's
_Archiv_, p. 367, 1826).
He appreciated the morphological idea of the serial repetition of parts,
and gave it accurate formulation. The whole vertebrate body, he
considered, was composed of a longitudinal series of _morphological
elements_, each of which was made up a section from each of the
fundamental organs--a vertebra, a section of the nerve-cord, and so on
(_Entwickelungsgeschichte_, ii., p. 53). Groups of these morphological
elements formed _morphological divisions_, such as the vertebral
segments of the head with their highly developed neural arches, or the
segments of the neck with their undeveloped haemal arches. The
morphological elements are clearly shown only in the animal parts, but
there are indications in the embryo of a segmentation also of the
vegetative parts,--the gill-slits, for instance, and the vascular
arches. The vegetative parts, however, develop on the whole
unsymmetrically (_cf._ Bichat). These elements which von Baer
distinguishes are morphological units, as he himself points out,
contrasting them with organs which are not usually units in a
morphological sense. "We call organ," he writes, "each part that has by
reason of its form or its function a certain distinctiveness, but this
concept is very indefinite, and possesses, from a morphological point of
view, little value. For this reason it seems necessary to introduce into
scientific morphology the concepts of morphological elements and
divisions" (ii., p. 84).
Von Baer exercised a very considerable influence upon the subsequent
trend of morphological theory. By his criticism of the Meckel-Serres
theory, he rid morphology for a time of an idea which was leading it
astray; by his substitution of the law that development is always from
the general to the special, he set morphologists looking for the
archetype in the embryo, not in the adult alone, and made them realise
that homologies could often best be sought in the earliest stages of
development; by formulating the germ-layer theory he supplied
morphologists with a new criterion of homology, based upon the special
relations of the parts (germ-layers) which are first differentiated in
all development. He made the study of development an essential part of
morphology.
[166] _De generatione Animalium_.
[167] _De formato foetu_, ? 1600; _De formatione
foetus_, 1604.
[168] _Exercitationes de generatione animalium_, 1651.
[169] _De formatione pulli in ovo_, 1673; _De ovo
incubato_, 1686.
[170] _De formatione pulli in ovo_, 1757-8; _Sur la
formation du coeur dans le poulet_, 1758.
[171] _Theoria generatioinis_, 1759; _De formatione
intestinorum_, 1768-9.
[172] _Beitraege zur Entwickelung des Huehnchens im Ei._
Wuerzburg, 1818. Also in Latin in shorter form, 1817.
[173] _Untersuchungen ue. die Entwickelungsgeschichte der
Fische_; Leipzig, 1835.
[174] Cuvier, in 1812, _Ann. Mus. d'Hist. Nat._, xix.; von
Baer in 1816, _Nova Acta Acad. Nat. Cur._ See
_Entwickelungsgeschichte der Thiere_, i., p. vii., f.n.
[175] Compare a parallel passage in Prevost et Dumas:--"At
the very first sight one will be struck with the
resemblance between the forms of the very early embryos
of these two classes, a resemblance so extraordinary
that one cannot refuse to admit the conclusions
resulting from it. The resemblance is so striking that
one can defy the most experienced observer to
distinguish in any way the embryos of dog or rabbit ...
from those of fowls or ducks of a corresponding
age."--_Ann. Sci. nat._, iii., p. 132, 1824.
[176] _De l'organisation des Animaux_, i., p. 140, 1822.
[177] "Ueber das aeussere und innere Skelet," Meckel's
_Archiv fuer Anat. u. Physiol._, pp. 327-76, 1826. See,
too, his _Entwickelungsgeschichte_, i., pp. 181, ff.
[178] Von Baer wrote an appreciative biography of Cuvier,
published posthumously in 1897, _Lebensgeschichte
Cuviers_, ed. L. Stieda. French trans. in _Ann. Sci.
Nat._ (_Zool._), ix., 1907.
[179] Cuvier et Valenciennes, _Histoire naturelle des
Poissons_, i., p. 550.
[180] _Mem. Mus. d'Hist. Nat._, iii., pp. 98-119, 1817.
[181] _Lecons d'Anatomie comparee_, 3rd ed., vol. i., p.
414, Bruxelles, 1836.
[182] In the aforementioned paper in Mueller's _Archiv_ he
criticises Carus vigorously and is sarcastic on
Geoffroy.