easily seen. It would appear, indeed, that it is of a very delicate
texture and readily destroyed by reagents. It is for this reason that
the layers of the Vertebrate embryo are commonly represented as being
quite isolated from one another, and that the medullary canal is
nearly always represented as being completely isolated at certain
stages from the surrounding tissues. In reality the layers are all
connected together by this delicate tissue--in a sparse form, it is
true--which not only extends between them, but also in a denser and
more distinct form pervades them. In the germinal layers themselves,
and in the organs developing from them, this tissue is in the young
stages almost entirely obscured by the densely packed nuclei which it
contains. For instance, in the wall of the medullary canal in the
Vertebrate embryo, in the splanchnic and somatic layers of mesoderm of
the same embryo, and in the developing nerve cords of the _Peripatus_
embryo, the nuclei are at first so densely crowded together that it is
almost impossible to see the protoplasmic framework in which they
rest, but as development proceeds this extra-nuclear tissue becomes
more largely developed, and the nuclei are forced apart, so that it
becomes visible and receives various names according to its position.
In the wall of the medullary canal of the Vertebrate embryo, on the
outside of which it becomes especially conspicuous in certain places,
and on the dorsal side of the developing nerve cords of the
_Peripatus_ embryo, it constitutes the white matter of the developing
nerve cord; in the mesoblastic tissue outside, where it at the same
time becomes more conspicuous (Sedgwick, "Monograph of the Development
of _Peripatus capensis_," _Studies from the Morph. Lab. of the
University of Cambridge_, iv., 1889, p. 131), it forms the looser
network of the mesoblastic reticulum; and connecting the two, in place
of the few and delicate strands of this tissue of the former stage,
there are at certain places well-marked cords of a relatively dense
texture, with the meshes of the reticulum elongated in the direction
of the cord. This latter structure is an incipient nerve trunk. It can
be traced outwards into the mesoblastic reticulum, from the strands of
which it is indeed developed, and with which it is continuous not only
at its free end, but also along its whole course. In this way the
nerve trunks are developed--by a gathering up, so to speak, of the
fibres of the reticulum into bundles. These bundles are generally
marked by the possession of nuclei, especially in their cortical
parts, which become no doubt the nuclei of the nerve sheath, and, in
the neighbourhood of the ganglia, of nerve cells. From this account of
the early development of the nerves, it is apparent that they are in
their origin continuous with all the other tissues of the body, with
that of the central nervous system and with that which becomes
transformed into muscular tissue and connective and epithelial
tissues. All these tissues are developed from the general reticulum,
which in the young embryo can be seen to pervade the whole body, not
being confined to the mesoderm, but extending between the nuclei of
the ectoderm and endoderm, and forming the extra-nuclear, so-called
cellular, protoplasm of those layers. Moreover, it must be remarked
that in the stages of the embryo with which we are here concerned the
so-called cellular constitution of the tissues, which is such a marked
feature of the older embryo and adult, has not been arrived at. It is
true, indications of it may be seen in some of the earlier-formed
epithelia, but of nerve cells, muscular cells, and many kinds of gland
cells no distinct signs are yet visible. This remark particularly
applies to nerve cells, which do not make their appearance until a
much later stage--not, indeed, until some time after the principal
nerve trunks and ganglia are indicated as tracts of pale fibrous
substance and aggregations of nuclei respectively.
The embryos of Elasmobranchs--particularly of _Scyllium_--are the best
objects in which to study the development of nerves. In many embryos
it is difficult to make out what happens, because the various parts of
the body remain so close together that the process is obscured, and
the loosening of the mesoblastic nuclei is deferred until after the
nerves have begun to be differentiated. The process may also be traced
in the embryos of _Peripatus_, where the main features are essentially
similar to those above described (op. cit. p. 131). The development of
the motor nerves has been worked out in _Lepidosiren_ by J. Graham
Kerr (_Trans. Roy. Soc. of Edinburgh_, 41, 1904. p. 119).
To sum up, the development of nerves is not, as has been recently urged,
an outgrowth of cell processes from certain cells, but is a
differentiation of a substance which was already in position, and from
which all other organs of the body have been and are developed. It
frequently happens that the young nerve tracts can be seen sooner near
the central organ than elsewhere, but it is doubtful if any importance
can be attached to this fact, since it is not constantly observed. For
instance, in the case of the third nerve of _Scyllium_ the
differentiation appears to take place earliest near the ciliary
ganglion, and to proceed from that point to the base of the mid-brain.
Coelom.
There are two main methods in which new organs are developed. In the
one, which indicates the possibility of physiological continuity, the
organ arises by the direct modification of a portion of a pre-existing
organ; the development of the central nervous system of the Vertebrata
from a groove in the embryonic ectoderm may be taken as an example of
this method. In the other method there is no continuity which can be in
any way interpreted as physiological; a centre of growth appears in one
of the parts of the embryo, and gives rise to a mass of tissue which
gradually shapes itself into the required organ. The development of the
central nervous system in Teleosteans and in other similar exceptional
cases may be mentioned as an example of the second plan. Such a centre
of growth is frequently called a blastema, and consists of a mass of
closely packed nuclei which have arisen by the growth-activity of the
nuclei in the neighbourhood. The coelom, an organ which is found in the
so-called coelomate animals, and which in the adult is usually divided
up more or less completely into three parts, namely, body-cavity, renal
organs, generative glands, presents in different animals both these
methods of development. In certain animals it develops by the direct
modification of a part of the primitive enteron, while in others it
arises by the gradual shaping of a mass of tissue which consists of a
compact mass of nuclei derived by nuclear proliferation from one or more
of the pre-existing tissues of the body. Inasmuch as the first rudiment
of the coelom nearly always makes its appearance at an early stage, when
the ectoderm and endoderm are almost the only tissues present, and as it
then bulks relatively very large and frequently contains within itself
the potential centres of growth of other organs, e.g. mesenchymal organs
(see above), it has come to be regarded by embryologists as being the
forerunner of all the so-called mesodermal organs of the body, and has
been dignified with the somewhat mysterious rank which attaches to the
conception of a germinal layer. Its prominence and importance at an
early stage led embryologists, as has already been explained, to
overlook the fact that although some of the centres of growth for the
formation of other non-coelomic mesodermal organs and tissues may be
contained within it, all are not so contained, and that there are
centres of mesodermal growth still left in the ectoderm and endoderm
after its establishment. If these considerations, and others like them,
are correct, it would seem to follow that the conception implied by the
word mesoderm has no objective existence, that the tissue of the embryo
called mesoderm, though sometimes mainly the rudiment of the coelom, is
often much more than this, and contains within itself the rudiment of
many, sometimes of all, of the organs appertaining to the mesenchyme. In
thus containing within itself the potential centres of growth of other
organs and tissues which are commonly ranked as mesodermal, it is not
different from the rudiments of the two other organs already formed,
namely, the ectoderm and endoderm; for these contain within themselves
centres of growth for the production of so-called mesodermal tissues, as
witness the nerve-crest of Vertebrata, the growing-point of the
pronephric duct, and the formation of blood-vessels from the hypoblast
described for some members of the same group.
In Echinodermata, _Amphioxus_, Enteropneusta, and a few other groups,
the coelom develops from a portion or portions of the primitive enteron,
which eventually becomes separated from the rest and forms a variable
number of closed sacs lying between the gut and the ectoderm. The number
of these sacs varies in different animals, but the evidence at present
available seems to show that the maximum number is five--an unpaired one
in front and two pairs behind--and, further, that if a less number of
sacs is actually separated from the enteron, the rule is for these sacs
so to divide up that they give rise to five sacs arranged in the manner
indicated. The Enteropneusta present us with the clearest case of the
separation of five sacs from the primitive enteron (W. Bateson, _Quart.
Journ. Mic. Sci._ xxiv., 1884). In _Amphioxus_, according to the
important researches of E.W. MacBride (_Quart. Journ. Mic. Sci._ xl.
589), it appears that a similar process occurs, though it is complicated
by the fact that the sacs of the posterior pair become divided up at an
early stage into many pairs. In _Phoronis_ there are indications of the
same phenomenon (A.T. Masterman, _Quart. Journ. Mic. Sci._ xliii. 375).
In the Chaetognatha a single sac only is separated from the enteron, but
soon becomes divided up. In the Brachiopoda one pair of sacs is
separated from the enteron, but our knowledge of their later history is
not sufficient to enable us to say whether they divide up into the
typically arranged five sacs. In Echinodermata the number of sacs
separated from the enteron varies from one to three; but though the
history of these shows considerable differences, there are reasons to
believe that the typical final arrangement is one unpaired and two
paired sacs. But however many sacs may arise from the primitive enteron,
and however these sacs may ultimately divide up and arrange themselves,
the important point of development common to all these animals, about
which there can be no dispute, is that the coelom is a direct
differentiation of a portion of the enteron.
In the majority of the Coelomata the coelomic rudiment does not arise by
the simple differentiation of a pre-existing organ, and there is
considerable variation in its method of formation. Speaking generally,
it may be said to arise by the differentiation of a blastema (see
above), which develops at an early stage as a nuclear proliferation from
one or more growth-centres in one or both of the primary layers. It
appears in this tissue as a sac or as a series of sacs, which become
transformed into the body-cavity (except in the Arthropoda), into the
renal organs (with the possible exception, again, of some Arthropoda),
and into the reproductive glands. In metamerically segmented animals the
appearance of the cavities of these sacs is synchronous with, and
indeed determines, the appearance of metameric segmentation. In all
segmented animals in which the mesoderm (coelomic rudiment) appears as a
continuous sheet or band of tissue on each side of the body, the
coelomic cavity makes its first appearance not as a continuous space on
each side, which later becomes divided up into the structures called
mesoblastic somites, but as a series of paired spaces round which the
coelomic tissue arranges itself in an epithelial manner. In the
Vertebrata, it is true, the ventral portion of the coelom appears at
first as a continuous space, at any rate behind the region of the two
anterior pairs of somites, but in the dorsal portion the coelomic cavity
is developed in the usual way, the coelomic tissue becoming transformed
into the muscle plates and rudimentary renal tubules of the later
stages. With regard to this ventral portion of the coelom in Vertebrata,
it is to be noticed that the cavity in it never becomes divided up, but
always remains continuous, forming the perivisceral portion of the
coelom. The probable explanation of this peculiarity in the development
of the Vertebrate coelom, as compared with that of _Amphioxus_ and other
segmented animals, is that the segmented stage of the ventral portion of
the coelom is omitted. This explanation derives some support from the
fact that even in animals in which the coelom is at its first appearance
wholly segmented, it frequently happens that in the adult the
perivisceral portion of it is unsegmented, i.e. it loses during
development the segmentation which it at first possesses. This happens
in many Annelida and in _Amphioxus_. The lesson, then, which the early
history of the coelom in segmented animals teaches is, that however the
coelomic cavity first makes its appearance, whether by evaginations from
the primitive enteron, or by the hollowing out of a solid blastema-like
tissue which has developed from one or both of the primary layers, it is
in its first origin segmented, and forms the basis on which the segments
of the adult are moulded. In Arthropoda the origin of the coelom is
similar to that of Annelids, but its history is not completely known in
any group, with the exception of _Peripatus_. In this genus it develops
no perivisceral portion, as in other groups, but gives rise solely to
the nephridia and to the reproductive organs. It is probable, though not
certainly proved, that the history of the coelom in other Arthropods is
essentially similar to that of _Peripatus_, allowance being made for the
fact that the nephridial portion does not attain full development in
those forms which are without nephridia in the adult.
With regard to the development of the vascular system, little can be
said here, except that it appears to arise from the spaces of the
mesoblastic reticulum. When this reticulum is sparse or so delicate as
to give way in manipulation, these spaces appear to be represented by a
continuous space which in the earliest stages of development is
frequently spoken of as the blastocoel or segmentation cavity. They
acquire special epithelial walls, and form the main trunks and network
of smaller vessels found in animals with a canalicular vascular system,
or the large sinus-like spaces characteristic of animals with a
haemocoelic body-cavity.
Transient embryonic organs.
The existence of a phase at the beginning of life during which a young
animal acquires its equipment by a process of growth of the germ is of
course intelligible enough; such a phase is seen in the formation of
buds, and in the sexual reproduction of both animals and plants. The
remarkable point is that while in most cases this embryonic growth is a
direct and simple process--e.g. animal and plant buds, embryonic
development of plant seeds--in many cases of sexual reproduction of
animals it is not direct, and the embryonic phase shows stages of
structure which seem to possess a meaning other than that of being
merely phases of growth. The fact that these stages of structure through
which the embryo passes sometimes present for a short time features
which are permanent in other members of the same group, adds very
largely to the interest of the phenomenon and necessitates its careful
examination. This may be divided into two heads: (1) in relation to
embryos, (2) in relation to larvae. So far as embryos are concerned, we
shall limit ourselves mainly to a consideration of the Vertebrata,
because in them are found most instances of that remarkable phenomenon,
the temporary assumption by certain organs of the embryo of stages of
structure which are permanent in other members of the same group. As is
well known, the embryos of the higher Vertebrata possess in the
structure of the pharynx and of the heart and vascular system certain
features--namely, paired pharyngeal apertures, a simple tubular heart,
and a single ventral aorta giving off right and left a number of
branches which pass between the pharyngeal apertures--which permanently
characterize those organs in fishes. The skeleton, largely bony in the
adult, passes through a stage in which it is entirely without bone, and
consists mainly of cartilage--the form which it permanently possesses in
certain fishes. Further, the Vertebrate embryo possesses for a time a
notochord, a segmented muscular system, a continuity between the
pericardium and the posterior part of the perivisceral cavity--all
features which characterize certain groups of Pisces in the adult state.
Instances of this kind might be multiplied, for the work of anatomists
and embryologists has of late years been largely devoted to adding to
them. Examples of embryonic characters which are not found in the adults
of other Vertebrates are the following:--At a certain stage of
development the central nervous system has the form of a groove in the
skin, there is a communication at the hind end of the body between the
neural and alimentary canals, the mouth aperture has at first the form
of an elongated slit, the growing end of the Wolffian duct is in some
groups continuous with the ectoderm, and the retina is at one stage a
portion of the wall of the medullary canal. In the embryos of the lower
Vertebrates many other instances of the same interesting character might
be mentioned; for instance, the presence of a coelomic sac close to the
eye, of another in the jaw, and of a third near the ear (Elasmobranchs),
the opening of the Mullerian duct into the front end of the Wolffian
duct, and the presence of an aperture of communication between the
muscle-plate coelom and the nephridial coelom.
Recapitulation theory.
The interest attaching to these remarkable facts is much increased by
the explanation which has been given of them. That explanation, which is
a deduction from the theory of evolution, is to the effect that the
peculiar embryonic structures and relations just mentioned are due to
the retention by the embryo of features which, once possessed by the
adult ancestor, have been lost in the course of evolution. This
explanation, which at once suggests itself when we are dealing with
structures actually present in adult members of other groups, does not
so obviously apply to those features which are found in no adult animal
whatsoever. Nevertheless it has been extended to them, because they are
of a nature which it is not impossible to suppose might have existed in
a working animal. Now this explanation, which, it will be observed, can
only be entertained on the assumption that the evolution theory is true,
has been still further extended by embryologists in a remarkable and
frequently unjustifiable manner, and has been applied to all embryonic
processes, finally leading to the so-called recapitulation theory, which
asserts that embryonic history is a shortened recapitulation of
ancestral history, or, to use the language of modern zoology, that the
_ontogeny_ or development of the individual contains an abbreviated
record of the _phylogeny_ or development of the race. A theory so
important and far-reaching as this requires very careful examination.
When we come to look for the facts upon which it is based, we find that
they are non-existent, for the ancestors of all living animals are dead,
and we have no means of knowing what they were like. It is true there
are fossil remains of animals which have lived, but these are so
imperfect as to be practically useless for the present requirements.
Moreover, if they were perfectly preserved, there would be no evidence
to show that they were ancestors of the animals now living. They might
have been animals which have become extinct and left no descendants.
Thus the explanation ordinarily given of the embryonic structures
referred to is purely a deduction from the evolution theory. Indeed, it
is even less than this, for all that can be said is something of this
kind: if the evolution theory is true, then it in conceivable that the
reason why the embryo of a bird passes through a stage in which its
pharynx presents some resemblance to that of a fish is that a remote
ancestor of the bird possessed a pharynx with lateral apertures such as
are at present found in fishes.
But the explanation is sometimes pushed even further, and it is said
that these pharyngeal apertures of the ancestral bird had the same
respiratory function as the corresponding structures in modern fishes.
That this is going too far a little reflection will show. For if it be
admitted that all so-called vestigial structures had once the same
function as the homologous structures when fully developed in other
animals, it becomes necessary to admit that male mammals must once have
had fully developed mammary glands and suckled the young, that female
mammals formerly were provided with a functional penis, and that in
species in which the females have a trace of the secondary sexual
characters of the male the latter were once common to both sexes. The
second and more extended form of the explanation plainly introduces a
considerable amount of contentious matter, and it will be advisable, in
the first instance, at any rate, to confine ourselves to a critical
examination of the less ambitious conception. This explanation obviously
implies the view that in the course of evolution the tendency has been
for structures to persist in the embryo after they have been lost in the
adult. Is there any justification for this view? It is clearly
impossible to get any direct evidence, because, as explained above, we
have no knowledge of the ancestors of living animals; but if we assume
the evolution theory to be true, there is a certain amount of indirect
evidence which is distinctly opposed to the view. As is well known,
living birds are without teeth, but it is generally assumed that their
edentulous condition has been comparatively recently acquired, and that
they are descended from animals which, at a time not very remote from
the present, possessed teeth. Considering the resemblance of birds to
other terrestrial vertebrates, and the fact that extinct birds, not
greatly differing from birds now living, are known to have had teeth, it
must be allowed that there is some warrant for the assumption. Yet in no
single case has it been certainly shown that any trace of teeth has been
developed in the embryo. The same remark applies to a large number of
similar cases; for instance, the reduced digits of the bird's hand and
foot and the limbs of snakes. Moreover, organs which are supposed to
have become recently reduced and functionless in the adult are also
reduced in the embryo; for instance, digits 3 and 4 of the horse's foot,
the hind limbs of whales (G.A. Guldberg and F. Nansen, "On the
Development and Structure of Whales," _Bergen Museum_, 1894), the
spiracle of Elasmobranchii. In fact, considerations of this kind
distinctly point to the view that any tendency to the reduction or
enlargement of an organ in the adult is shared approximately to the same
extent by the embryo. But there are undoubtedly some, though not many,
cases in which organs which were presumably present in an ancestral
adult have persisted in the embryo of the modern form. As an instance
may be mentioned the presence in whale-bone whales of imperfectly formed
teeth, which are absorbed comparatively early in foetal life (Julin,
_Arch. biologie_, i., 1880, p. 75).
It therefore becomes necessary to inquire why in some cases an organ is
retained by the embryo after its loss by the adult, whereas in other
cases it dwindles and presumably disappears simultaneously in the embryo
and the adult. The whole question is examined and discussed by the
present writer in the _Quarterly Journal of Microscopical Science_,
xxxvi., 1894, p. 35, and the conclusions there reached are as
follows:--A disappearing adult organ is not retained in a relatively
greater development by an organism in the earlier stages of its
individual growth unless it is of functional importance to the young
form. In cases in which the whole development is embryonic this rarely
happens, because the conditions of embryonic life are so different from
free life that functional embryonic organs are usually organs _sui
generis, e.g._ the placenta, amnion, &c., which cannot be traced to a
modification of organs previously present in the adult. It does,
however, appear to have happened sometimes, and as an instance of it
may be mentioned the _ductus arteriosus_ of the Sauropsidan and
Mammalian embryo. On the other hand, when there is a considerable period
of larval life, it does appear that there is a strong case for thinking
that organs which have been lost by the adult may be retained and made
use of by the larva. The best-known example that can be given of this is
the tadpole of the frog. Here we find organs, viz. gills and gill-slits,
which are universally regarded as having been attributes of all
terrestrial Vertebrata in an earlier and aquatic condition, and we also
notice that their retention is due to their being useful on account of
the supposed ancient conditions of life having been retained. Many other
instances, more or less plausible, of a like retention of ancestral
features by larvae might be mentioned, and it must be conceded that
there are strong reasons for supposing that larvae often retain traces,
more or less complete, of ancestral stages of structure. But this
admission does not carry with it any obligation to accept the widely
prevalent view that larval history can in any way be regarded as a
recapitulation of ancestral history. Far from it, for larvae in
retaining some ancestral features are in no way different from adults;
they only differ from adults in the features which they have retained.
Both larvae and adults retain ancestral features, and both have been
modified by an adaptation to their respective conditions of life which
has ever been becoming more perfect.
The conclusion, then, has been reached, that whereas larvae frequently
retain traces of ancestral stages of adult structure, embryos will
rarely do so; and we are confronted again with the question, How are we
to account for the presence in the embryo of numerous functionless
organs which cannot be explained otherwise than as having been inherited
from a previous condition in which they were functional? The answer is
that the only organs of this kind which have been retained are organs
which have been retained by the larvae of the ancestors after they have
been lost by the adult, and have become in this way impressed upon the
development. As an illustration taken from current natural history of
the manner in which larval characters are in actual process of becoming
embryonic may be mentioned the case of the viviparous salamander
(_Salamander atra_), in which the gills, &c., are all developed but
never used, the animal being born without them. In other and closely
allied species of salamander there is a considerable period of larval
life in which the gills and gill-slits are functional, but in this
species the larval stage, for the existence of which there was a
distinct reason, viz. the entirely aquatic habits of life in the young
state, has become at one stroke embryonic by its simple absorption into
the embryonic period. The view, then, that embryonic development is
essentially a recapitulation of ancestral history must be given up; it
contains only a few references to ancestral history, namely, those which
have been preserved probably in a much modified form by previous larvae.
Law of v. Baer.
We must now pass to the consideration of another supposed law of
embryology--the so-called law of v. Baer. This generalization is usually
stated as follows:--Embryos of different species of the same group are
more alike than adults, and the resemblances are greater the younger the
embryo examined. Great importance has been attached to this
generalization by embryologists and naturalists, and it is very widely
accepted. Nevertheless, it is open to serious criticism. If it were
true, we should expect to find that embryos of closely similar species
would be indistinguishable, but this is notoriously not the case. On the
contrary, they often differ more than do the adults, in support of which
statement the embryos of the different species of _Peripatus_ may be
referred to. The generalization undoubtedly had its origin in the fact
that there is what may be called a family resemblance between embryos,
but this resemblance, which is by no means exact, is purely superficial,
and does not extend to anatomical detail. On the contrary, it may be
fairly argued that in some cases embryos of widely dissimilar members of
the same group present anatomical differences of a higher morphological
value than do the adults (see Sedgwick, _loc. cit_.), and, as stated
above the embryos of closely allied animals are distinguishable at all
stages of development, though the distinguishing features are not the
same as those which distinguish the adults. To say that the development
of the organism and of its component parts is a progress from the simple
to the complex is to state a truism, but to state that it is also a
progress from the general to the special is to go altogether beyond the
facts. The bipinnaria larva of an echinoderm, the trochosphere larva of
an annelid, the blastodermic vesicle of a mammal are all as highly
specialized as their respective adults, but the specialization is for a
different purpose, and of a different kind to that which characterizes
the adult.
History of embryology.
In its scientific and systematic form embryology may be considered as
having only taken birth within the last century, although the germ from
which it sprung was already formed nearly half a century earlier. The
ancients, it is true, as we see by the writings of Aristotle and Galen,
pursued the subject with interest, and the indefatigable Greek
naturalist and philosopher had even made continued series of
observations on the progressive stages of development in the incubated
egg, and on the reproduction of various animals; but although, after the
revival of learning, various anatomists and physiologists from time to
time made contributions to the knowledge of the foetal structure in its
larger organs, yet from the minuteness of the observations required for
embryological research, it was not till the microscope came into use for
the investigation of organic structure that any intimate knowledge was
attained of the nature of organogenesis. It is not to be wondered at,
therefore, that during a long period, in this as in other branches of
physical inquiry, vague speculations took the place of direct
observation and more solid information. This is apparent in most of the
works treating of generation during the 16th and part of the 17th
centuries.[2]
Harvey was the first to give, in the middle of the latter century, a new
life and direction to investigation of this subject, by his discovery of
the connexion between the cicatricula of the yolk and the rudiments of
the chick, and by his faithful description of the successive stages of
development as observed in the incubated egg, as well as of the progress
of gestation in some Mammalia. He had also the merit of fixing the
attention of physiologists upon general laws of development as deduced
from actual observation of the phenomena, by the enunciation of two
important propositions, viz.--(1) that all animals are produced out of
ova, and (2) that the organs of the embryo arise by new formation, or
_epigenesis,_ and not by mere enlargement out of a pre-existing
invisible condition (_Exercitationes de generatione animalium_,
Amstelodami, 1651). Harvey's observations, however, were aided only by
the use of magnifying glasses (perspecillae), probably of no great
power, and he saw nothing of the earliest appearances of the embryo in
the first thirty-six hours, and believed the blood and the heart to be
the parts first formed.
The influence of the work of Harvey, and of the successful application
of the microscope to embryological investigation, was soon afterwards
apparent in the admirable researches of Malpighi of Bologna, as evinced
by his communications to the Royal Society of London in 1672, "De ovo
incubato," and "De formatione pulli," and more especially in his
delineations of some of the earlier phenomena of development, in which,
as in many other parts of minute anatomy, he partially or wholly
anticipated discoveries, the full development of which has only been
accomplished in the present century. Malpighi traced the origin of the
embryo almost to its very commencement in the formation of the
cerebro-spinal groove within the cicatricula, which he removed from the
opaque mass of the yolk; and he only erred in supposing the embryonal
rudiments to have pre-existed as such in the egg, in consequence,
apparently, of his having employed for observation, in very warm
weather, eggs which, though he believed them to be unincubated, had in
reality undergone some of the earlier developmental changes.
The works of Walter Needham (1667), Regnier de Graaf (1673), Swammerdam
(1685), Vallisneri (1689)--following upon those of Harvey--all contain
important contributions to the knowledge of our subject, as tending to
show the similarity in the mode of production from ova in a variety of
animals with that previously best known in birds. The observations more
especially of de Graaf, Nicolas Steno and J. van Horne gave much greater
precision to the knowledge of the connexion between the origin of the
ovum of quadrupeds and the vesicles of the ovary now termed Graafian,
which de Graaf showed always burst and discharged their contents on the
occurrence of pregnancy.
These observations bring us to the period of Boerhaave and Albinus in
the earlier part of the 18th century, and in the succeeding years to
that of Haller, whose vast erudition and varied and accurate original
observations threw light upon the entire process of reproduction in
animals, and brought its history into a more systematic and intelligible
form. A considerable part of the seventh and the whole of the eighth
volumes of Haller's great work, the _Elementa physiologiae_, published
at successive times from 1757 to 1766, are occupied with the general
view of the function of generation, while his special contributions to
embryology are contained in his _Deux memoires sur la formation du coeur
dans le poulet_ and _Deux memoires sur la formation des os_, both
published at Lausanne in 1758, and republished in an extended and
altered form, together with his "Observations on the early condition of
the Embryo in Quadrupeds," made along with Kuhlemann, in the _Opera
minora_ (1762-1768). Though originally educated as a believer in the
doctrine of "preformation" by his teacher Boerhaave, Haller was soon led
to abandon that view in favour of "epigenesis" or new formation, as may
be seen in various parts of his works published before the middle of the
century; see especially a long note explanatory of the grounds of his
change of opinion in his edition of Boerhaave's _Praelectiones
academicae_, vol. v. part 2, p. 497 (1744), and his _Primae lineae
physiologiae_ (1747). But some years later, and after having been
engaged in observing the phenomena of development in the incubated egg,
he again changed his views, and during the remainder of his life was a
keen opponent of the system of epigenesis, and a defender and exponent
of the theory of "evolution," as it was then named--a theory very
different from that now bearing the name, and which implied belief in
the pre-existence of the organs of the embryo in the germ, according to
the theory of encasement (_emboitement_) or inclusion supported by
Leibnitz and Bonnet. (See the interesting work of Bonnet,
_Considerations sur les corps organises_, Amsterdam, 1762, for an
account of his own views and those of Haller.)
It was reserved for Caspar Frederick Wolff (1733-1794), a German by
birth, but naturalized afterwards in Russia, to bring forward
observations which, though almost entirely neglected for a long time
after their publication, and in some measure discredited under the
influence of Haller's authority, were sixty years later acknowledged to
have established the theory of epigenesis upon the secure basis of
ascertained facts, and to have laid the first foundation of the
morphological science of embryology. Wolff's work, entitled _Theoria
generationis_, first published as an inaugural Dissertation at Berlin in
1759, was republished with additions in German at Berlin in 1764, and
again in Latin at Halle in 1774. Wolff also wrote a "Memoir on the
Development of the Intestine" in _Nov. comment. acad. Petropol_., 1768
and 1769. But it was not till the latter work was translated into German
by J.F. Meckel, and appeared in his _Archiv_ for 1812, that Wolff's
peculiar merits as the founder of modern embryology came to be known or
fully appreciated.
The special novelty of Wolff's discoveries consisted mainly in this,
that he showed that the germinal part of the bird's egg forms a layer of
united granules or organized particles (cells of the modern
histologist), presenting at first no semblance of the form or structure
of the future embryo, but gradually converted by various morphological
changes in the formative material, which are all capable of being traced
by observation, into the several rudimentary organs and systems of the
embryo. The earlier form of the embryo he delineated with accuracy; the
actual mode of formation he traced in more than one organ, as for
example in the alimentary canal, and he was the discoverer of several
new and important embryological facts, as in the instance of the
primordial kidneys, which have thus been named the Wolffian bodies.
Wolff further showed that the growing parts of plants owe their origin
to organized particles or cells, so that he was led to the great
generalization that the processes of embryonic formation and of adult
growth and nutrition are all of a like nature in both plants and
animals. No advance, however, was made upon the basis of Wolff's
discoveries till the year 1817, when the researches of C.H. Pander on
the development of the chick gave a fuller and more exact view of the
phenomena less clearly indicated by Wolff, and laid down with greater
precision a plan of the formation of parts in the embryo of birds, which
may be regarded as the foundation of the views of all subsequent
embryologists.
But although the minuter investigation of the nature and true theory of
the process of embryonic development was thus held in abeyance for more
than half a century, the interval was not unproductive of observations
having an important bearing on the knowledge of the anatomy of the
foetus and the function of reproduction. The great work of William
Hunter on the human gravid uterus, containing unequalled pictorial
illustrations of its subject from the pencil of Rymsdyk and other
artists, was published in 1775;[3] and during a large part of the same
period numerous communications to the _Memoirs_ of the Royal Society
testified to the activity and genius of his brother, John Hunter, in the
investigation of various parts of comparative embryology. But it is
mainly in his rich museum, and in the manuscripts and drawings which he
left, and which have been in part described and published in the
catalogue of his wonderful collection, that we obtain any adequate idea
of the unexampled industry and wide scope of research of that great
anatomist and physiologist.
As belonging to a somewhat later period, but still before the time when
the more strict investigation of embryological phenomena was resumed by
Pander, there fall to be noticed, as indicative of the rapid progress
that was making, the experiments of L. Spallanzani, 1789; the researches
of J.H. von Autenrieth, 1797, and of Soemmering, 1799, on the human
foetus; the observations of Senff on the formation of the skeleton,
1801; those of L. Oken and D.G. Kieser on the intestine and other
organs, 1806; Oken's remarkable work on the bones of the head, 1807
(with the views promulgated in which Goethe's name is also intimately
connected); J.F. Meckel's numerous and valuable contributions to
embryology and comparative anatomy, extending over a long series of
years; and F. Tiedemann's classical work on the development of the
brain, 1816.
The observations of the Russian naturalist, Christian Heinrich Pander
(1794-1865), were made at the instance and under the immediate
supervision of Prof. Dollinger at Wurzburg, and we learn from von Baer's
autobiography that he, being an early friend of Pander's, and knowing his
qualifications for the task, had pointed him out to Dollinger as well
fitted to carry out the investigation of development which that professor
was desirous of having accomplished. Pander's inaugural dissertation was
entitled _Historia metamorphoseos quam ovum incubatum prioribus quinque
diebus subit_ (Virceburgi, 1817); and it was also published in German
under the title of _Beitrage zur Entwickelungsgeschichte des Huhnchens im
Eie_ (Wurzburg, 1817). The beautiful plates illustrating the latter work
were executed by the elder E.J. d'Alton, well known for his skill in
scientific observation, delineation and engraving.
Pander observed the germinal membrane or _blastoderm_, as he for the
first time called it, of the fowl's egg to acquire three layers of
organized substance in the earlier period of incubation. These he named
respectively the serous or outer, the vascular or middle, and the mucous
or inner layers; and he traced with great skill and care the origin of
the principal rudimentary organs and systems from each of these layers,
pointing out shortly, but much more distinctly than Wolff had done, the
actual nature of the changes occurring in the process of development.
Karl Ernest von Baer (q.v.), the greatest of modern embryologists,
was, as already remarked, the early friend of Pander, and, at the time
when the latter was engaged in his researches at Wurzburg, was
associated with Dollinger as prosector, and engaged with him in the
study of comparative anatomy. He witnessed, therefore, though he did not
actually take part in, Pander's researches; and the latter having
afterwards abandoned the inquiry, von Baer took it up for himself in the
year 1819, when he had obtained an appointment in the university of
Konigsberg, where he was the colleague of Burdach and Rathke, both of
whom were able coadjutors in the investigation of the subject of his
choice. (See v. Baer's interesting autobiography, published on his
retirement from St Petersburg to Dorpat in 1864.)
Von Baer's observations were carried on at various times from 1819 to
1826 and 1827, when he published the first results in a description of
the development of the chick in the first edition of Burdach's
_Physiology_.
It was at this time that von Baer made the important discovery of the
ovarian ovum of mammals and of man, totally unknown before his time, and
was thus able to prove as matter of exact observation what had only been
surmised previously, viz. the entire similarity in the mode of origin of
these animals with others lower in the scale. (_Epistola de ovi
mammalium et hominis genesi,_ Lipsiae, 1827. See also the interesting
commentary on or supplement to the _Epistola_ in Heusinger's Journal,
and the translation in Breschet's _Repertoire_, Paris, 1829.)
In 1829 von Baer published the first part of his great work, entitled
_Beobachtungen und Reflexionen uber die Entwickelungsgeschichte der
Thiere_, the second part of which, still leaving the work incomplete,
did not appear till 1838. In this work, distinguished by the fulness,
richness and extreme accuracy of the observations and descriptions, as
well as by the breadth and soundness of the general views on embryology
and allied branches of biology which it presents, he gave a detailed
account not only of the whole progress of development of the chick as
observed day by day during the incubation of the egg, but he also
described what was known, and what he himself had investigated by
numerous and varied observations, of the whole course of formation of
the young in other vertebrate animals. His work is in fact a system of
comparative embryology, replete with new discoveries in almost every
part.
Von Baer's account of the layers of the blastoderm differs somewhat from
that of Pander, and appears to be more consistent with the further
researches which have lately been made than was at one time supposed, in
this respect, that he distinguished from a very early period two
primitive or fundamental layers, viz. the animal or upper, and the
vegetative or lower, from each of which, in connexion with two
intermediate layers derived from them, the fundamental organs and
systems of the embryo are derived:--the animal layer, with its
derivative, supplying the dermal, neural, osseous and muscular; the
vegetative layer, with its derivative, the vascular and mucous
(intestinal) systems. He laid down the general morphological principle
that the fundamental organs have essentially the shape of tubular
cavities, as appears in the first form of the central organ of the
nervous system, in the two muscular and osseous tubes which form the
walls of the body, and in the intestinal canal; and he followed out with
admirable clearness the steps by which from these fundamental systems
the other organs arise secondarily, such as the organs of sense, the
glands, lungs, heart, vascular glands, Wolffian bodies, kidneys and
generative organs.
To complete von Baer's system there was mainly wanting a more minute
knowledge of the intimate structure of the elementary tissues, but this
had not yet been acquired by biologists, and it remained for Theodor
Schwann of Liege in 1839, along with whom should be mentioned those who,
like Robert Brown and M.J. Schleiden, prepared the way for his great
discovery, to point out the uniformity in histological structure of the
simpler forms of plants and animals, the nature of the organized animal
and vegetable cell, the cellular constitution of the primitive ovum of
animals, and the derivation of the various tissues, complex as well as
simple, from the transformation or, as it is now called, differentiation
of simple cellular elements,--discoveries which have exercised a
powerful and lasting influence on the whole progress of biological
knowledge in our time, and have contributed in an eminent degree to
promote the advance of embryology itself.
To K.B. Reichert of Berlin more particularly is due the first
application of the newer histological views to the explanation of the
phenomena of development, 1840. To him and to R.A. von Kolliker and R.
Virchow is due the ascertainment of the general principle that there is
no free-cell formation in embryonic development and growth, but that all
organs are derived from the multiplication, combination and
transformation of cells, and that all cells giving rise to organs are
the descendants or progeny of previously existing cells, and that these
may be traced back to the original cell or cell-substance of the ovum.
It may be that modern research has somewhat modified the views taken by
biologists of the statements of Schwann as to the constitution of the
organized cell, especially as regards its simplest or most elementary
form, and has indicated more exactly the nature of the protoplasmic
material which constitutes its living basis; but it has not caused any
very wide departure from the general principles enunciated by that
physiologist. Schwann's treatise, entitled _Microscopical Researches
into the Accordance in the Structure and Growths of Animals and Plants_,
was published in German at Berlin in 1839, and was translated into
English by Henry Smith, and printed for the Sydenham Society in 1847,
along with a translation of Schleiden's memoir, "Contributions to
Phytogenesis," which originally appeared in 1838 in Muller's _Archiv_
for that year, and which had also been published in English in Taylor
and Francis's _Scientific Memoirs_, vol. ii. part vi.
Among the newer observations of the same period which contributed to a
more exact knowledge of the structure of the ovum itself may be
mentioned--first the discovery of the germinal vesicle, or nucleus, in
the germ-disk of birds by J.E. von Purkinje (_Symbolae ad ovi avium
historiam ante incubationem_, Vratislaviae, 1825, and republished at
Leipzig in 1830); second, von Baer's discovery of the mammiferous ovum
in 1827, already referred to; third, the discovery of the germinal
vesicle of mammals by J.V. Coste in 1834, and its independent
observation by Wharton Jones in 1835; and fourth, the observation in the
same year by Rudolph Wagner of the germinal macula or nucleus. Coste's
discovery of the germinal vesicle of Mammalia was first communicated to
the public in the _Comptes rendus_ of the French Academy for 1833, and
was more fully described in the _Recherches sur la generation des
mammiferes_, by Delpech and Coste (Paris, 1834). Thomas Wharton Jones's
observations, made in the autumn of 1834, without a knowledge of Coste's
communication, were presented to the Royal Society in 1835. This
discovery was also confirmed and extended by G.G. Valentin and Bernardt,
as recorded by the latter in his work _Symb. ad ovi mammal. hist. ante
praegnationem_. Rudolph Wagner's observations first appeared in his
_Textbook of Comparative Anatomy_, published at Leipzig in 1834-1835,
and in Muller's _Archiv_ for the latter year. His more extended
researches are described in his work _Prodromus hist. generationis
hominis atque animalium_ (Leipzig, 1836), and in a memoir inserted in
the _Trans. of the Roy. Bavarian Acad. of Sciences_ (Munich, 1837).
The two decades of years from 1820 to 1840 were peculiarly fertile in
contributions to the anatomy of the foetus and the progress of
embryological knowledge. The researches of Prevost and Dumas on the ova
and primary stages of development of Batrachia, birds and mammals, made
as early as 1824, deserve especial notice as important steps in advance,
both in the discovery of the process of yolk segmentation in the
batrachian ovum, and in their having shown almost with the force of
demonstration, previous to the discovery of the mammiferous ovarian ovum
by von Baer, that that body must exist as a minute spherule in the
Graafian follicle of the ovary, although they did not actually succeed
in bringing the ova clearly under observation.
The works of Pockels (1825), of Seiler (1831), of G. Breschet (1832), of
A.A.L.M. Velpeau (1833), of T.L.W. Bischoff (1834)--all bearing upon
human embryology; the researches of Coste in comparative embryology in
1834, already referred to, and those published by the same author in
1837; the publication of Johannes Muller's great work on physiology, and
Rudolph Wagner's smaller text-book, in both of which the subject of
embryology received a very full treatment, together with the excellent
_Manual of the Development of the Foetus_, by Valentin, in 1835, the
first separate and systematic work on the whole subject, now secured to
embryology its permanent place among the biological sciences on the
Continent; while in this country attention was drawn to the subject by
the memoirs of Allen Thomson (1831), Th. Wharton Jones (1835-1838) and
Martin Barry (1839-1840).
Among the more remarkable special discoveries which belong to the period
now referred to, a few may be mentioned, as, for example, that of the
chorda dorsalis by von Baer, a most important one, which may be regarded
as the key to the whole of vertebral morphology; the phenomenon of yolk
segmentation, now known to be universal among animals, but which was
only first carefully observed in Batrachia by Prevost and Dumas (though
previously casually noticed by Swammerdam), and was soon afterwards
followed out by Rusconi and von Baer in fishes; the discovery of the
branchial clefts, plates and vascular arches in the embryos of the
higher abranchiate animals by H. Rathke in 1825-1827; the able
investigation of the transformations of these arches by Reichert in
1837; and the researches on the origin and development of the urinary
and generative organs by Johannes Muller in 1829-1830.
On entering the fifth decade of the 19th century, the number of original
contributions and systematic treatises becomes so great as to render the
attempt to enumerate even a selection of the more important of them
quite unsuitable to the limits of the present article. We must be
satisfied, therefore, with a reference to one or two which seem to stand
out with greater prominence than the rest as landmarks in the progress
of embryological discovery. Among these may first be mentioned the
researches of Theodor L.W. von Bischoff, formerly of Giessen and later
of Munich, on the development of the ovum in Mammalia, in which a series
of the most laborious, minute and accurate observations furnished a
greatly novel and very full history of the formative process in several
animals of that class. These researches are contained in four memoirs,
treating separately of the development of the rabbit, the dog, the
guinea-pig and the roe-deer, and appeared in succession in the years
1842, 1845, 1852 and 1854.
Next may be mentioned the great work of Coste, entitled _Histoire gen.
et particul. du developpement des animaux_, of which, however, only four
fasciculi appeared between the years 1847 and 1859, leaving the work
incomplete. In this work, in the large folio form, beautiful
representations are given of the author's valuable observations on human
embryology, and on that of various mammals, birds and fishes, and of the
author's discovery in 1847 of the process of partial yolk segmentation
in the germinal disk of the fowl's egg during its descent through the
oviduct, and his observations on the same phenomenon in fishes and
mammals.
The development of reptiles received important elucidation from the
researches of Rathke, in his history of the development of serpents,
published at Konigsberg in 1839, and in a similar work on the turtle in
1848, as well as in a later one on the crocodile in 1866, along with
which may be associated the observations of H.J. Clark on the
"Embryology of the Turtle," published in Agassiz's _Contributions to
Natural History, &c._, 1857.
The phenomena of yolk segmentation, to which reference has more than
once been made, and to which later researches give more and more
importance in connexion with the fundamental phenomena of development,
received great elucidation during this period, first from the
observations of C.T.E. von Siebold and those of Bagge on the complete
yolk segmentation of the egg in nematoid worms in 1841, and more fully
by the observations of Kolliker in the same animals in 1843. The nature
of partial segmentation of the yolk was first made known by Kolliker in
his work on the development of the Cephalopoda in 1844, and, as has
already been mentioned, the phenomena were observed by Coste in the eggs
of birds. The latter observations have since been confirmed by those of
Oellacher, Gotte and Kolliker. Further researches in a vast number of
animals give every reason to believe that the phenomenon of segmentation
is in some shape or other the invariable precursor of embryonic
formation.
The first considerable work on the development of a division of the
invertebrates was that of Maurice Herold of Marburg on spiders, _De
generatione aranearum ex ovo_, published at Marburg in 1824, in which
the whole phenomena of the formative processes in that animal are
described with remarkable clearness and completeness. A few years later
an important series of contributions to the history of the development
of invertebrate animals appeared in the second volume of Burdach's work
on _Physiology_, of which the first edition was published in 1828, and
in this the history of the development of the Entozoa was the production
of Ch. Theod. von Siebold, and that of most of the other invertebrates
was compiled by H. Rathke from the results of his own observations and
those of others. These memoirs, together with others subsequently
published by Rathke, notably that _Uber die Bildung und
Entwickelungsgeschichte d. Flusskrebses_ (Leipzig, 1829), in which an
attempt is made to extend the doctrine of the derivation of the organs
from the germinal layers to the invertebrata, entitle him to be regarded
as the founder of invertebrate embryology.
A large body of facts having by this time been ascertained with respect
to the more obvious processes of development, a further attempt to refer
the phenomena of organogenesis to morphological and histological
principles became desirable. More especially was the need felt to point
out with greater minuteness and accuracy the relation in which the
origin of the fundamental organs of the embryo stands to the layers of
the blastoderm; and this we find accomplished with signal success in the
researches of R. Remak on the development of the chick and frog,
published between the years 1850 and 1855.
Starting from Pander's discovery of the trilaminate blastoderm, Remak
worked out the development of the chick in the light of the cell-theory
of Schleiden and Schwann. He observed the division of the middle layer
into two by a split which subsequently gives rise to the body-cavity
(pleuro-peritoneal space) of the adult; and traced the principal organs
which came from these two layers (_Hautfaserblatt_ and _Darmfaserblatt_)
respectively. In this manner the foundations of the germ-layer theory
were established in their modern form.
A great step forward was made in 1859 by T.H. Huxley, who compared the
serous and mucous layers of Pander with the ectoderm and endoderm of the
Coelenterata. But in spite of this comparison it was generally held that
germinal layers similar to those of the vertebrata were not found in
invertebrate animals, and it was not until the publication in 1871 of
Kowalewsky's researches (see below) that the germinal layer theory was
applied to the embryos of all the Metazoa. But the year 1859 will be for
ever memorable in the history of science as the year of the publication
of the _Origin of Species_. If the enunciation of the cell-theory may be
said to have marked a first from a second period in the history of
embryology, the publication of Darwin's great idea ushered in a third.
Whereas hitherto the facts of anatomy and development were loosely held
together by the theory of types which owed its origin and maintenance to
Cuvier, L. Agassiz, J. Muller and R. Owen, they were now combined into
one organic whole by the theory of descent and by the hypothesis of
recapitulation which was deduced from that theory. First clearly
enunciated by Johann Muller in his well-known work _Fur Darwin_
published in 1864 (rendered in England as _Facts for Darwin_, 1869), the
view that a knowledge of embryonic and larval histories would lay bare
the secrets of race history and enable the course of evolution to be
traced and so lead to the discovery of the natural system of
classification, gave a powerful stimulus to embryological research. The
first fruits of this impetus were gathered by Alexander Agassiz, A.
Kowalewsky and E. Metschnikoff. Agassiz, in his memoir on the
_Embryology of the Starfish_ published in 1864, showed that the
body-cavity in Echinodermata arises as a differentiation of the enteron
of the larva and so laid the foundations of our present knowledge of the
coelom. This discovery was confirmed in 1869 by Metschnikoff ("Studien
ub. d. Entwick. d. Echinodermen u. Nemertinen," _Mem. Ac. Petersbourg_
(7), 41, 1869), and extended by him to Tornaria, the larva of
_Balanoglossus_ in 1870 ("Untersuchungen ub. d. Metamorphose einiger
Seethiere," _Zeit. f. wiss. Zoologie_, 20, 1870). In 1871 Kowalewsky in
his classical memoir, entitled "Embryologische Studien an Wurmern und
Arthropoden" (_Mem. Acad. Petersbourg_ (7), 16, 1871), proved the same
fact for Sagitta and added immensely to our knowledge of the early
stages of development of the Invertebrata. These memoirs formed the
basis on which subsequent workers took their stand. Amongst the most
important of these was F.M. Balfour (1851-1882). Led to the study of
embryology by his teacher, M. Foster, in association with whom he
published in 1874 the Elements of Embryology, Balfour was one of the
first to take advantage of the facilities for research offered by Dr. A.
Dohrn's Zoological Station at Naples which has since become so
celebrated. Here he did the work which was subsequently published in
1878 in his _Monograph of the Development of Elasmobranch Fishes_, and
which constituted the most important addition to vertebrate morphology
since the days of Johannes Muller. This was followed in 1879 and 1881 by
the publication of his _Treatise on Comparative Embryology_, the first
work in which the facts of the rapidly growing science were clearly and
philosophically put together, and the greatest. The influence of
Balfour's work on embryology was immense and is still felt. He was an
active worker in every department of it, and there are few groups of the
animal kingdom on which he has not left the impress of his genius.
In the period under consideration the output of embryological work has
been enormous. No group of the animal kingdom has escaped exhaustive
examination, and no effort has been spared to obtain the embryos of
isolated and out of the way forms, the development of which might have a
bearing upon important questions of phylogeny and classification. Of
this work it is impossible to speak in detail in this summary. It is
only possible to call attention to some of its more important features,
to mention the more important advances, and to refer to some of the more
striking memoirs.
Marine zoological stations have been established, expeditions have been
sent to distant countries, and the methods of investigation have been
greatly improved. Since Anton Dohrn founded the Stazione Zoologica at
Naples in 1872, observatories for the study of marine organisms have
been established in most countries. Of journeys which have been made to
distant countries and which have resulted in important contributions to
embryology, may be mentioned the expedition (1884-1886) of the cousins
Sarasin to Ceylon (development of Gymnophiona), of E. Selenka to Brazil
and the East Indies (development of Marsupials, Primates and other
mammals, 1877, 1889, 1892), of A.A.W. Hubrecht to the East Indies (1890,
development of _Tarsius_), of W.H. Caldwell to Australia (1883-1884,
discovery of the nature of the ovum and oviposition of _Echidna_ and of
_Ceratodus_), of A. Sedgwick to the Cape (1883, development of
_Peripatus_), of J. Graham Kerr to Paraguay (1896, development of
_Lepidosiren_), of R. Semon to Australia and the Malay Archipelago
(1891-1893, development of Monotremata, Marsupialia), and of J.S.
Budgett to Africa (1898, 1900, 1901, 1903, development of _Polypterus_).
In methods, while great improvements have been made in the processes of
hardening and staining embryos, the principal advance has been the
introduction in 1883 by W.H. Caldwell in his work on the development of
_Phoronis_ of the method of making tape-worm like strings of sections as
a result of which the process of mounting in order all the sections
obtained from an embryo was much facilitated, and the use of an
automatic microtome rendered possible. The method of Golgi for the
investigation of the nervous system, introduced in 1875, must also be
mentioned here.
The word "coelom" (q.v.) was introduced into zoology by E. Haeckel in
1872 (_Kalkschwamme_, p. 468) as a convenient term for the body-cavity
(pleuro-peritoneal). The word was generally adopted, and was applied
alike to the blood-containing body-cavity of Arthropods and to the
body-cavity of Vertebrata and segmented worms, in which there is no
blood. In 1875 Huxley (_Quarterly Journ. of Mic. Science_, 15, p. 53),
relying on the researches of Agassiz, Metschnikoff and Kowalewsky above
mentioned, put forward the idea that according to their development
three kinds of body-cavity ought to be distinguished: (1) the
enterocoelic which arises from enteric diverticula, (2) the schizocoelic
which develops as a split in the embryonic mesoblast, and (3) the
epicoelic which was enclosed by folds of the skin and lined by ectoderm
(e.g. atrial cavity of Tunicates, &c.). This suggestion was of great
importance, because it led the embryologists of the day (Balfour, the
brothers Hertwig, Lankester and others) to discuss the question as to
whether there was not more than one kind of body-cavity. The Hertwigs
(_Coelomtheorie_, Jena, 1881) distinguished two kinds, the enterocoel
and the pseudocoel. The former, to which they limited the use of the
word coelom, and which is developed directly or indirectly from the
enteron, is found in Annelida, Arthropoda, Echinodermata, Chordata, &c.
The latter they regarded as something quite different from the coelom
and as arising by a split in what they called for the first time
mesenchyme; the mesenchyme being the non-epithelial mesoderm, which they
described as consisting of amoeboid cells, but which we now know to
consist of a continuous reticulum. The next step was made by E. Ray
Lankester, who in 1884 (_Zoologischer Anzeiger_) showed that the
pericardium of Mollusca does not contain blood, and therein differs from
the rest of the body-cavity which does contain blood, but no suggestion
is made that the blood-containing space is not coelomic. In fact it was
generally held by the anatomists of the day that the coelom and the
vascular system were different parts of the same primitive organ, though
separate from it in the adult except in Arthropoda and Mollusca. In the
Mollusca, it is true, the pericardial part of the coelom was held to be
separate from the vascular, and the Hertwigs had reached the correct
conception that the pericardium of these animals was alone true coelom,
the vascular part being pseudocoel. This was the state of morphological
opinion until 1886, when it was shown (_Proc. Cambridge Phil. Soc._, 6,
1886, p. 27) (1) that the coelom of _Peripatus_ gives rise to the
nephridia and generative glands only, and to no other part of the
body-cavity of the adult, (2) that the nephridia of the adult do not
open as had been supposed into the body-cavity, (3) that the body-cavity
is entirely formed of the blood-containing space, the coelom having no
perivisceral portion. These results were extended by the same author
(_Quart. Journ. Mic. Sci._, 27, 1887, pp. 486-540) to other Arthropods
and to the Mollusca, and the modern theory of the coelom was finally
established. An increased precision was given to the conception of
coelom by the discovery in 1880 (_Quart. Journ. Mic. Sci._, 20, p. 164)
that the nephridia of Elasmobranchs are a direct differentiation of a
portion of it. In 1886 this was extended to _Peripatus_ (_Proc. Camb.
Phil. Soc._, 6, p. 27) and doubtless holds universally.
In 1864 it was suggested by V. Hensen (Virchow's _Archiv_, 31) that the
rudiments of nerve-fibres are present from the beginning of development
as persistent remains of connexions between the incompletely separated
cells of the segmented ovum. This suggestion fell to the ground because
it was held by embryologists that the cleavage of the ovum resulted in
the formation of completely separate cells, and that the connexions
between the adult cells were secondary. In 1886 it was shown (_Quarterly
Journ. Mic. Sci._, 26, p. 182) that in _Peripatus Capensis_ the cells of
the segmenting ovum do not separate from one another, but remain
connected by a loose protoplasmic network. This discovery has since been
extended to other ova, even to the small so-called holoblastic ova, and
a basis of fact was found for Hensen's suggestion as to the embryonic
origin of nerves (_Quart. Journ. Mic. Sci._, 33, 1892, pp. 581-584). An
extension and further application of the new views as to the cell-theory
and the embryonic origin of nerves thus necessitated was made in 1894
(_Quart. Journ. Mic. Sci._, 37, p. 87), and in 1904 J. Graham Kerr
showed that the motor nerves in the dipnoan fish Lepidosiren arise in an
essentially similar manner (_Trans. Roy. Society of Edinburgh_, 41, p.
119).
In 1883 Elie Metschnikoff published his researches on the intracellular
digestion of invertebrates (_Arbeiten a. d. zoologischen Inst. Wien_, 5;
and _Biologisches Centralblatt_, 3, p. 560); these formed the basis of
his theory of inflammation and phagocytosis, which has had such an
important influence on pathology. As he himself has told us, he was led
to make these investigations by his precedent researches on the
development of sponges and other invertebrates. To quote his own words:
"Having long studied the problem of the germinal layers in the animal
series, I sought to give some idea of their origin and significance. The
part played by the ectoderm and endoderm appeared quite clear, and the
former might reasonably be regarded as the cutaneous investment of
primitive multicellular animals, while the latter might be regarded as
their organ of digestion. The discovery of intracellular digestion in
many of the lower animals led me to regard this phenomenon as
characteristic of those ancestral animals from which might be derived
all the known types of the animal kingdom (excepting, of course, the
Protozoa). The origin and part played by the mesoderm appeared the most
obscure. Thus certain embryologists supposed that this layer
corresponded to the reproductive organs of primitive animals: others
regarded it as the prototype of the organs of locomotion. My
embryological and physiological studies on sponges led me to the
conclusion that the mesoderm must function in the hypothetically
primitive animals as a mass of digestive cells, in all points similar to
those of the endoderm. This hypothesis necessarily attracted my
attention to the power of seizing foreign corpuscles possessed by the
mesodermic cells" (_Immunity in Infective Diseases_, English
translation, Cambridge, 1905).
The branch of embryology which concerns itself with the study of the
origin, history and conjugation of the individuals (gametes) which are
concerned in the reproduction of the species has made great advances.
These began in 1875 and following years with a careful examination of
the behaviour of the germinal vesicle in the maturation and
fertilization of the ovum. The history of the polar bodies, the origin
of the female pronucleus, the presence in the ovum of a second nucleus,
the male pronucleus, which gave rise to the first segmentation nucleus
by fusion with the female pronucleus, were discovered (E. van Beneden,
O. Butschli, O. Hertwig, H. Fol), and in 1876 O. Hertwig
(_Morphologisches Jahrbuch_, 3, 1876) for the first time observed the
entrance of a spermatozoon into the egg and the formation of the male
pronucleus from it. The centrosome was discovered by W. Flemming in 1875
in the egg of the fresh-water mussel, and independently in 1876 by E.
van Beneden in Dicyemids. In 1883 came E. van Beneden's celebrated
discovery (_Arch. Biologie_, 4) of the reduction of the number of
chromosomes in the nucleus of both male and female gametes, and of the
fact that the male and female pronuclei contribute the same number of
chromosomes to the zygote-nucleus. He also showed that the gametogenesis
in the male is a similar process to that in the female, and paved the
way for the acceptation of the view (due to Butschli) that polar bodies
are aborted female gametes. These discoveries were extended and
completed by subsequent workers, among whom may be mentioned E. van
Beneden, J.B. Carnoy, G. Platner, T. Boveri, O. Hertwig, A. Brauer. The
subject is still being actively pursued, and hopes are entertained that
some relation may be found between the behaviour of the chromosomes and
the facts of heredity.
Since 1874 (W. His, _Unsere Korperform und das physiologische Problem
ihrer Entstehung_) a new branch of embryology, which concerns itself
with the physiology of development, has arisen (experimental
embryology). The principal workers in this field have been W. Roux, who
in 1894 founded the _Archiv fur Entwickelungsmechanik der Organismen_,
T. Boveri and Y. Delage who discovered and elucidated the phenomenon of
merogony, J. Loeb who discovered artificial parthenogenesis, O. and R.
Hertwig, H. Driesch, C. Herbst, E. Maupas, A. Weismann, T.H. Morgan,
C.B. Davenport (_Experimental Morphology_, 2 vols., 1899) and many
others.
In the elucidation of remarkable life-histories we may point in the
first place to the work of A. Kowalewsky on the development of the
Tunicata ("Entwickelungsgeschichte d. einfachen Ascidien," _Mem. Acad.
Petersbourg_ (7), 10, 1866, and _Arch. f. Mic. Anatomie_, 7, 1871), in
which was demonstrated for the first time the vertebrate relationship of
the Tunicata (possession of a notochord, method of development of the
central nervous system) and which led to the establishment of the group
Chordata. We may also mention the work of Y. Delage in the metamorphosis
of _Sacculina_ (_Arch. zool. exp._ (2) 2, 1884), A. Giard (_Comptes
rendus_, 123, 1896, p. 836) and of A. Malaquin on _Monstrilla_ (_Arch.
zool. exp._ (3), 9, p. 81, 1901), of Delage (_Comptes rendus_, 103,
1886, p. 698) and Grassi and Calandruccio (_Rend. Acc. Lincei_ (5), 6,
1897, p. 43), on the development of the eels, and of P. Pergande on the
life-history of the Aphidae (_Bull. U.S. Dep. Agric. Ent._, technical
series, 9, 1901). The work of C. Grobben (_Arbeiten zool. Inst. Wien_,
4, 1882) and of B. Uljanin ("Die Arten der Gattung Doliolum," _Fauna u.
Flora des Golfes von Neapel_, 1884) on the extraordinary life-history
and migration of the buds in _Doliolum_ must also be mentioned. In pure
embryological morphology we have had Heymons' elucidation of the
Arthropod head, the work of Hatschek on Annelid and other larvae, the
works of H. Bury and of E.W. MacBride which have marked a distinct
advance in our knowledge of the development of Echinodermata, of K.
Mitsukuri, who has founded since 1882 an important school of embryology
in Japan, on the early development of Chelonia and Aves, of A. Brauer
and G.C. Price on the development of vertebrate excretory organs, of Th.
W. Bischoff, E. van Beneden, E. Selenka, A.A.W. Hubrecht, R. Bonnet, F.
Keibel and R. Assheton on the development of mammals, of A.A.W. Hubrecht
and E. Selenka on the early development and placentation of the
Primates, of J. Graham Kerr and of J.S. Budgett on the development of
Dipnoan and Ganoid fishes, of A. Kowalewsky, B. Hatschek, A. Willey and
E.W. MacBride on the development of Amphioxus, of B. Dean on the
development of Bdellostoma, of A. Gotte on the development of Amphibia,
of H. Strahl and L. Will on the early development of reptiles, of T.H.
Huxley, C. Gegenbaur and W.K. Parker on the development of the
vertebrate skeleton, of van Wijhe on the segmentation of the vertebrate
head, by which the modern theory of head-segmentation, previously
adumbrated by Balfour, was first established, of Leche and Rose on the
development of mammalian dentitions. We may also specially notice W.
Bateson's work on the development of _Balanoglossus_ and his inclusion
of this genus among the Chordata (1884), the discovery by J.P. Hill of a
placenta in the marsupial genus _Perameles_ (1895), the work of P.
Marchal (1904) on the asexual increase by fission of the early embryos
of certain parasitic Hymenoptera (so called germinogony), a phenomenon
which had been long ago shown to occur in _Lumbricus trapezoides_ by N.
Kleinenberg (1879) and by S.F. Harmer in Polyzoa (1893). The work on
cell-lineage which has been so actively pursued in America may be
mentioned here. It has consisted mainly of an extension of the early
work of A. Kowalewsky and B. Hatschek on the formation of the layers,
being a more minute and detailed examination of the origin of the
embryonic tissues.
The most important text-books and summaries which have appeared in
this period have been Korschelt and Heider's _Lehrbuch der
vergleichenden Entwickelungsgeschichte der wirbellosen Tiere_
(1890-1902), C.S. Minot's _Human Embryology_ (1892), and the _Handbuch
der vergleichenden und experimentellen Entwickelungslehre der
Wirbeltiere_, edited by O. Hertwig (1901, et seq.). See also K.E. von
Baer, _Uber Entwicklungsgeschichte der Tiere_ (Konigsberg, 1828,
1837); F.M. Balfour, _A Monograph on the Development of Elasmobranch
Fishes_ (London, 1878); _A Treatise on Comparative Embryology_, vols.
i. and ii. (London, 1885) (still the most important work on Vertebrate
Embryology); M. Duval, _Atlas d'Embryologie_ (Paris, 1889); M. Foster
and F.M. Balfour, _Elements of Embryology_ (London, 1883); O. Hertwig,
_Lehrbuch der Entwicklungsgeschichte des Menschen u. der Wirbeltiere_
(6th ed., Jena, 1898); A. Kolliker, _Entwicklungsgeschichte des
Menschen u. der hoheren Tiere_ (Leipzig, 1879); A.M. Marshall,
_Vertebrate Embryology_ (London, 1893). (A. Se.*)