We may distinguish three chief periods in the growth of our science of
human embryology. The first has been considered in the preceding
chapter; it embraces the whole of the preparatory period of research,
and extends from Aristotle to Caspar Friedrich Wolff, or to the year
1759, in which the epoch-making Theoria generationis was published.
The second period, with which we have now to deal, lasts about a
century--that is to say, until the appearance of Darwin's Origin of
Species, which brought about a change in the very foundations of
biology, and, in particular, of embryology. The third period begins
with Darwin. When we say that the second period lasted a full century,
we must remember that Wolff's work had remained almost unnoticed
during half the time--namely, until the year 1812. During the whole of
these fifty-three years not a single book that appeared followed up
the path that Wolff had opened, or extended his theory of embryonic
development. We merely find his views--perfectly correct views, based
on extensive observations of fact--mentioned here and there as
erroneous; their opponents, who adhered to the dominant theory of
preformation, did not even deign to reply to them. This unjust
treatment was chiefly due to the extraordinary authority of Albrecht
von Haller; it is one of the most astonishing instances of a great
authority, as such, preventing for a long time the recognition of
established facts.
The general ignorance of Wolff's work was so great that at the
beginning of the nineteenth century two scientists of Jena, Oken
(1806) and Kieser (1810), began independent research into the
development of the alimentary canal of the chick, and hit upon the
right clue to the embryonic puzzle, without knowing a word about
Wolff's important treatise on the same subject. They were treading in
his very footsteps without suspecting it. This can be easily proved
from the fact that they did not travel as far as Wolff. It was not
until Meckel translated into German Wolff's book on the alimentary
system, and pointed out its great importance, that the eyes of
anatomists and physiologists were suddenly opened. At once a number of
biologists instituted fresh embryological inquiries, and began to
confirm Wolff's theory of epigenesis.
This resuscitation of embryology and development of the
epigenesis-theory was chiefly connected with the university of
Wurtzburg. One of the professors there at that time was Dollinger, an
eminent biologist, and father of the famous Catholic historian who
later distinguished himself by his opposition to the new dogma of
papal infallibility. Dollinger was both a profound thinker and an
accurate observer. He took the keenest interest in embryology, and
worked at it a good deal. However, he is not himself responsible for
any important result in this field. In 1816 a young medical doctor,
whom we may at once designate as Wolff's chief successor, Karl Ernst
von Baer, came to Wurtzburg. Baer's conversations with Dollinger on
embryology led to a fresh series of most extensive investigations.
Dollinger had expressed a wish that some young scientist should begin
again under his guidance an independent inquiry into the development
of the chick during the hatching of the egg. As neither he nor Baer
had money enough to pay for an incubator and the proper control of the
experiments, and for a competent artist to illustrate the various
stages observed, the lead of the enterprise was given to Christian
Pander, a wealthy friend of Baer's who had been induced by Baer to
come to Wurtzburg. An able engraver, Dalton, was engaged to do the
copper-plates. In a short time the embryology of the chick, in which
Baer was taking the greatest indirect interest, was so far advanced
that Pander was able to sketch the main features of it on the ground
of Wolff's theory in the dissertation he published in 1817. He clearly
enunciated the theory of germinal layers which Wolff had anticipated,
and established the truth of Wolff's idea of a development of the
complicated systems of organs out of simple leaf-shaped primitive
structures. According to Pander, the leaf-shaped object in the hen's
egg divides, before the incubation has proceeded twelve hours, into
two different layers, an external serous layer and an internal mucous
layer; between the two there develops later a third layer, the
vascular (blood-vessel) layer.* (* The technical terms which are bound
to creep into this chapter will be fully understood later
on.--Translator.)
Karl Ernst von Baer, who had set afoot Pander's investigation, and had
shown the liveliest interest in it after Pander's departure from
Wurtzburg, began his own much more comprehensive research in 1819. He
published the mature result nine years afterwards in his famous work,
Animal Embryology: Observation and Reflection (not translated). This
classic work still remains a model of careful observation united to
profound philosophic speculation. The first part appeared in 1828, the
second in 1837. The book proved to be the foundation on which the
whole science of embryology has built down to our own day. It so far
surpassed its predecessors, and Pander in particular, that it has
become, after Wolff's work, the chief base of modern embryology.
Baer was one of the greatest scientists of the nineteenth century, and
exercised considerable influence on other branches of biology as well.
He built up the theory of germinal layers, as a whole and in detail,
so clearly and solidly that it has been the starting-point of
embryological research ever since. He taught that in all the
vertebrates first two and then four of these germinal layers are
formed; and that the earliest rudimentary organs of the body arise by
the conversion of these layers into tubes. He described the first
appearance of the vertebrate embryo, as it may be seen in the globular
yelk of the fertilised egg, as an oval disk which first divides into
two layers. From the upper or animal layer are developed all the
organs which accomplish the phenomena of animal life--the functions of
sensation and motion, and the covering of the body. From the lower or
vegetative layer come the organs which effect the vegetative life of
the organism--nutrition, digestion, blood-formation, respiration,
secretion, reproduction, etc.
Each of these original layers divides, according to Baer, into two
thinner and superimposed layers or plates. He calls the two plates of
the animal layer, the skin-stratum and muscle-stratum. From the upper
of these plates, the skin-stratum, the external skin, or outer
covering of the body, the central nervous system, and the
sense-organs, are formed. From the lower, or muscle-stratum, the
muscles, or fleshy parts and the bony skeleton--in a word, the motor
organs--are evolved. In the same way, Baer said, the lower or
vegetative layer splits into two plates, which he calls the
vascular-stratum and the mucous-stratum. From the outer of the two
(the vascular) the heart, blood-vessels, spleen, and the other
vascular glands, the kidneys, and sexual glands, are formed. From the
fourth or mucous layer, in fine, we get the internal and digestive
lining of the alimentary canal and all its dependencies, the liver,
lungs, salivary glands, etc. Baer had, in the main, correctly judged
the significance of these four secondary embryonic layers, and he
followed the conversion of them into the tube-shaped primitive organs
with great perspicacity. He first solved the difficult problem of the
transformation of this four-fold, flat, leaf-shaped, embryonic disk
into the complete vertebrate body, through the conversion of the
layers or plates into tubes. The flat leaves bend themselves in
obedience to certain laws of growth; the borders of the curling plates
approach nearer and nearer; until at last they come into actual
contact. Thus out of the flat gut-plate is formed a hollow gut-tube,
out of the flat spinal plate a hollow nerve-tube, from the skin-plate
a skin-tube, and so on.
Among the many great services which Baer rendered to embryology,
especially vertebrate embryology, we must not forget his discovery of
the human ovum. Earlier scientists had, as a rule, of course, assumed
that man developed out of an egg, like the other animals. In fact, the
preformation theory held that the germs of the whole of humanity were
stored already in Eve's ova. But the real ovum escaped detection until
the year 1827. This ovum is extremely small, being a tiny round
vesicle about the 1/120 of an inch in diameter; it can be seen under
very favourable circumstances with the naked eye as a tiny particle,
but is otherwise quite invisible. This particle is formed in the ovary
inside a much larger globule, which takes the name of the Graafian
follicle, from its discoverer, Graaf, and had previously been regarded
as the true ovum. However, in 1827 Baer proved that it was not the
real ovum, which is much smaller, and is contained within the
follicle. (Compare the end of Chapter 2.29.)
Baer was also the first to observe what is known as the segmentation
sphere of the vertebrate; that is to say, the round vesicle which
first develops out of the impregnated ovum, and the thin wall of which
is made up of a single layer of regular, polygonal (many-cornered)
cells (see the illustration in Chapter 1.12). Another discovery of his
that was of great importance in constructing the vertebrate stem and
the characteristic organisation of this extensive group (to which man
belongs) was the detection of the axial rod, or the chorda dorsalis.
There is a long, round, cylindrical rod of cartilage which runs down
the longer axis of the vertebrate embryo; it appears at an early
stage, and is the first sketch of the spinal column, the solid
skeletal axis of the vertebrate. In the lowest of the vertebrates, the
amphioxus, the internal skeleton consists only of this cord throughout
life. But even in the case of man and all the higher vertebrates it is
round this cord that the spinal column and the brain are afterwards
formed.
However, important as these and many other discoveries of Baer's were
in vertebrate embryology, his researches were even more influential,
from the circumstance that he was the first to employ the comparative
method in studying the development of the animal frame. Baer occupied
himself chiefly with the embryology of vertebrates (especially the
birds and fishes). But he by no means confined his attention to these,
gradually taking the various groups of the invertebrates into his
sphere of study. As the general result of his comparative
embryological research, Baer distinguished four different modes of
development and four corresponding groups in the animal world. These
chief groups or types are: 1, the vertebrata; 2, the articulata; 3,
the mollusca; and 4, all the lower groups which were then wrongly
comprehended under the general name of the radiata. Georges Cuvier had
been the first to formulate this distinction, in 1812. He showed that
these groups present specific differences in their whole internal
structure, and the connection and disposal of their systems of organs;
and that, on the other hand, all the animals of the same type--say,
the vertebrates--essentially agreed in their inner structure, in spite
of the greatest superficial differences. But Baer proved that these
four groups are also quite differently developed from the ovum; and
that the series of embryonic forms is the same throughout for animals
of the same type, but different in the case of other animals. Up to
that time the chief aim in the classification of the animal kingdom
was to arrange all the animals from lowest to highest, from the
infusorium to man, in one long and continuous series. The erroneous
idea prevailed nearly everywhere that there was one uninterrupted
chain of evolution from the lowest animal to the highest. Cuvier and
Baer proved that this view was false, and that we must distinguish
four totally different types of animals, on the ground of anatomic
structure and embryonic development.
Baer's epoch-making works aroused an extraordinary and widespread
interest in embryological research. Immediately afterwards we find a
great number of observers at work in the newly opened field, enlarging
it in a very short time with great energy by their various discoveries
in detail. Next to Baer's comes the admirable work of Heinrich Rathke,
of Konigsberg (died 1860); he made an extensive study of the
embryology, not only of the invertebrates (crustaceans, insects,
molluscs), but also, and particularly, of the vertebrates (fishes,
tortoises, serpents, crocodiles, etc.). We owe the first comprehensive
studies of mammal embryology to the careful research of Wilhelm
Bischoff, of Munich; his embryology of the rabbit (1840), the dog
(1842), the guinea-pig (1852), and the doe (1854), still form
classical studies. About the same time a great impetus was given to
the embryology of the invertebrates. The way was opened through this
obscure province by the studies of the famous Berlin zoologist,
Johannes Muller, on the echinoderms. He was followed by Albert
Kolliker, of Wurtzburg, writing on the cuttlefish (or the
cephalopods), Siebold and Huxley on worms and zoophytes, Fritz Muller
(Desterro) on the crustacea, Weismann on insects, and so on. The
number of workers in this field has greatly increased of late, and a
quantity of new and astonishing discoveries have been made. One
notices, in several of these recent works on embryology, that their
authors are too little acquainted with comparative anatomy and
classification. Palaeontology is, unfortunately, altogether neglected
by many of these new workers, although this interesting science
furnishes most important facts for phylogeny, and thus often proves of
very great service in ontogeny.
A very important advance was made in our science in 1839, when the
cellular theory was established, and a new field of inquiry bearing on
embryology was suddenly opened. When the famous botanist, M.
Schleiden, of Jena, showed in 1838, with the aid of the microscope,
that every plant was made up of innumerable elementary parts, which we
call cells, a pupil of Johannes Muller at Berlin, Theodor Schwann,
applied the discovery at once to the animal organism. He showed that
in the animal body as well, when we examine its tissues in the
microscope, we find these cells everywhere to be the elementary units.
All the different tissues of the organism, especially the very
dissimilar tissues of the nerves, muscles, bones, external skin,
mucous lining, etc., are originally formed out of cells; and this is
also true of all the tissues of the plant. These cells are separate
living beings; they are the citizens of the State which the entire
multicellular organism seems to be. This important discovery was bound
to be of service to embryology, as it raised a number of new
questions. What is the relation of the cells to the germinal layers?
Are the germinal layers composed of cells, and what is their relation
to the cells of the tissues that form later? How does the ovum stand
in the cellular theory? Is the ovum itself a cell, or is it composed
of cells? These important questions were now imposed on the
embryologist by the cellular theory.
The most notable effort to answer these questions--which were attacked
on all sides by different students--is contained in the famous work,
Inquiries into the Development of the Vertebrates (not translated) of
Robert Remak, of Berlin (1851). This gifted scientist succeeded in
mastering, by a complete reform of the science, the great difficulties
which the cellular theory had at first put in the way of embryology. A
Berlin anatomist, Carl Boguslaus Reichert, had already attempted to
explain the origin of the tissues. But this attempt was bound to
miscarry, since its not very clear-headed author lacked a sound
acquaintance with embryology and the cell theory, and even with the
structure and development of the tissue in particular. Remak at length
brought order into the dreadful confusion that Reichert had caused; he
gave a perfectly simple explanation of the origin of the tissues. In
his opinion the animal ovum is always a simple cell: the germinal
layers which develop out of it are always composed of cells; and these
cells that constitute the germinal layers arise simply from the
continuous and repeated cleaving (segmentation) of the original
solitary cell. It first divides into two and then into four cells; out
of these four cells are born eight, then sixteen, thirty-two, and so
on. Thus, in the embryonic development of every animal and plant there
is formed first of all out of the simple egg cell, by a repeated
subdivision, a cluster of cells, as Kolliker had already stated in
connection with the cephalopods in 1844. The cells of this group
spread themselves out flat and form leaves or plates; each of these
leaves is formed exclusively out of cells. The cells of different
layers assume different shapes, increase, and differentiate; and in
the end there is a further cleavage (differentiation) and division of
work of the cells within the layers, and from these all the different
tissues of the body proceed.
These are the simple foundations of histogeny, or the science that
treats of the development of the tissues (hista), as it was
established by Remak and Kolliker. Remak, in determining more closely
the part which the different germinal layers play in the formation of
the various tissues and organs, and in applying the theory of
evolution to the cells and the tissues they compose, raised the theory
of germinal layers, at least as far as it regards the vertebrates, to
a high degree of perfection.
Remak showed that three layers are formed out of the two germinal
layers which compose the first simple leaf-shaped structure of the
vertebrate body (or the "germinal disk"), as the lower layer splits
into two plates. These three layers have a very definite relation to
the various tissues. First of all, the cells which form the outer skin
of the body (the epidermis), with its various dependencies (hairs,
nails, etc.)--that is to say, the entire outer envelope of the
body--are developed out of the outer or upper layer; but there are
also developed in a curious way out of the same layer the cells which
form the central nervous system, the brain and the spinal cord. In the
second place, the inner or lower germinal layer gives rise only to the
cells which form the epithelium (the whole inner lining) of the
alimentary canal and all that depends on it (the lungs, liver,
pancreas, etc.), or the tissues that receive and prepare the
nourishment of the body. Finally, the middle layer gives rise to all
the other tissues of the body, the muscles, blood, bones, cartilage,
etc. Remak further proved that this middle layer, which he calls "the
motor-germinative layer," proceeds to subdivide into two secondary
layers. Thus we find once more the four layers which Baer had
indicated. Remak calls the outer secondary leaf of the middle layer
(Baer's "muscular layer") the "skin layer" (it would be better to say,
skin-fibre layer); it forms the outer wall of the body (the true skin,
the muscles, etc.). To the inner secondary leaf (Baer's "vascular
layer") he gave the name of the "alimentary-fibre layer"; this forms
the outer envelope of the alimentary canal, with the mesentery, the
heart, the blood-vessels, etc.
On this firm foundation provided by Remak for histogeny, or the
science of the formation of the tissues, our knowledge has been
gradually built up and enlarged in detail. There have been several
attempts to restrict and even destroy Remak's principles. The two
anatomists, Reichert (of Berlin) and Wilhelm His (of Leipzic),
especially, have endeavoured in their works to introduce a new
conception of the embryonic development of the vertebrate, according
to which the two primary germinal layers would not be the sole sources
of formation. But these efforts were so seriously marred by ignorance
of comparative anatomy, an imperfect acquaintance with ontogenesis,
and a complete neglect of phylogenesis, that they could not have more
than a passing success. We can only explain how these curious attacks
of Reichert and His came to be regarded for a time as advances by the
general lack of discrimination and of grasp of the true object of
embryology.
Wilhelm His published, in 1868, his extensive Researches into the
Earliest Form of the Vertebrate Body,* (* None of His's works have
been translated into English.) one of the curiosities of embryological
literature. The author imagines that he can build a "mechanical theory
of embryonic development" by merely giving an exact description of the
embryology of the chick, without any regard to comparative anatomy and
phylogeny, and thus falls into an error that is almost without
parallel in the history of biological literature. As the final result
of his laborious investigations, His tells us "that a comparatively
simple law of growth is the one essential thing in the first
development. Every formation, whether it consist in cleavage of
layers, or folding, or complete division, is a consequence of this
fundamental law." Unfortunately, he does not explain what this "law of
growth" is; just as other opponents of the theory of selection, who
would put in its place a great "law of evolution," omit to tell us
anything about the nature of this. Nevertheless, it is quite clear
from His's works that he imagines constructive Nature to be a sort of
skilful tailor. The ingenious operator succeeds in bringing into
existence, by "evolution," all the various forms of living things by
cutting up in different ways the germinal layers, bending and folding,
tugging and splitting, and so on.
His's embryological theories excited a good deal of interest at the
time of publication, and have evoked a fair amount of literature in
the last few decades. He professed to explain the most complicated
parts of organic construction (such as the development of the brain)
in the simplest way on mechanical principles, and to derive them
immediately from simple physical processes (such as unequal
distribution of strain in an elastic plate). It is quite true that a
mechanical or monistic explanation (or a reduction of natural
processes) is the ideal of modern science, and this ideal would be
realised if we could succeed in expressing these formative processes
in mathematical formulae. His has, therefore, inserted plenty of
numbers and measurements in his embryological works, and given them an
air of "exact" scholarship by putting in a quantity of mathematical
tables. Unfortunately, they are of no value, and do not help us in the
least in forming an "exact" acquaintance with the embryonic phenomena.
Indeed, they wander from the true path altogether by neglecting the
phylogenetic method; this, he thinks, is "a mere by-path," and is "not
necessary at all for the explanation of the facts of embryology,"
which are the direct consequence of physiological principles. What His
takes to be a simple physical process--for instance, the folding of
the germinal layers (in the formation of the medullary tube,
alimentary tube, etc.)--is, as a matter of fact, the direct result of
the growth of the various cells which form those organic structures;
but these growth-motions have themselves been transmitted by heredity
from parents and ancestors, and are only the hereditary repetition of
countless phylogenetic changes which have taken place for thousands of
years in the race-history of the said ancestors. Each of these
historical changes was, of course, originally due to adaptation; it
was, in other words, physiological, and reducible to mechanical
causes. But we have, naturally, no means of observing them now. It is
only by the hypotheses of the science of evolution that we can form an
approximate idea of the organic links in this historic chain.
All the best recent research in animal embryology has led to the
confirmation and development of Baer and Remak's theory of the
germinal layers. One of the most important advances in this direction
of late was the discovery that the two primary layers out of which is
built the body of all vertebrates (including man) are also present in
all the invertebrates, with the sole exception of the lowest group,
the unicellular protozoa. Huxley had detected them in the medusa in
1849. He showed that the two layers of cells from which the body of
this zoophyte is developed correspond, both morphologically and
physiologically, to the two original germinal layers of the
vertebrate. The outer layer, from which come the external skin and the
muscles, was then called by Allman (1853) the "ectoderm" (outer layer,
or skin); the inner layer, which forms the alimentary and reproductory
organs, was called the "entoderm" (= inner layer). In 1867 and the
following years the discovery of the germinal layers was extended to
other groups of the invertebrates. In particular, the indefatigable
Russian zoologist, Kowalevsky, found them in all the most diverse
sections of the invertebrates--the worms, tunicates, echinoderms,
molluscs, articulates, etc.
In my monograph on the sponges (1872) I proved that these two primary
germinal layers are also found in that group, and that they may be
traced from it right up to man, through all the various classes, in
identical form. This "homology of the two primary germinal layers"
extends through the whole of the metazoa, or tissue-forming animals;
that is to say, through the whole animal kingdom, with the one
exception of its lowest section, the unicellular beings, or protozoa.
These lowly organised animals do not form germinal layers, and
therefore do not succeed in forming true tissue. Their whole body
consists of a single cell (as is the case with the amoebae and
infusoria), or of a loose aggregation of only slightly differentiated
cells, though it may not even reach the full structure of a single
cell (as with the monera). But in all other animals the ovum first
grows into two primary layers, the outer or animal layer (the
ectoderm, epiblast, or ectoblast), and the inner or vegetal layer (the
entoderm, hypoblast, or endoblast); and from these the tissues and
organs are formed. The first and oldest organ of all these metazoa is
the primitive gut (or progaster) and its opening, the primitive mouth
(prostoma). The typical embryonic form of the metazoa, as it is
presented for a time by this simple structure of the two-layered body,
is called the gastrula; it is to be conceived as the hereditary
reproduction of some primitive common ancestor of the metazoa, which
we call the gastraea. This applies to the sponges and other zoophyta,
and to the worms, the mollusca, echinoderma, articulata, and
vertebrata. All these animals may be comprised under the general
heading of "gut animals," or metazoa, in contradistinction to the
gutless protozoa.
I have pointed out in my Study of the Gastraea Theory [not translated]
(1873) the important consequences of this conception in the morphology
and classification of the animal world. I also divided the realm of
metazoa into two great groups, the lower and higher metazoa. In the
first are comprised the coelenterata (also called zoophytes, or
plant-animals). In the lower forms of this group the body consists
throughout life merely of the primary germinal layers, with the cells
sometimes more and sometimes less differentiated. But with the higher
forms of the coelentarata (the corals, higher medusae, ctenophorae,
and platodes) a middle layer, or mesoderm, often of considerable size,
is developed between the other two layers; but blood and an internal
cavity are still lacking.
To the second great group of the metazoa I gave the name of the
coelomaria, or bilaterata (or the bilateral higher forms). They all
have a cavity within the body (coeloma), and most of them have blood
and blood-vessels. In this are comprised the six higher stems of the
animal kingdom, the annulata and their descendants, the mollusca,
echinoderma, articulata, tunicata, and vertebrata. In all these
bilateral organisms the two-sided body is formed out of four secondary
germinal layers, of which the inner two construct the wall of the
alimentary canal, and the outer two the wall of the body. Between the
two pairs of layers lies the cavity (coeloma).
Although I laid special stress on the great morphological importance
of this cavity in my Study of the Gastraea Theory, and endeavoured to
prove the significance of the four secondary germinal layers in the
organisation of the coelomaria, I was unable to deal satisfactorily
with the difficult question of the mode of their origin. This was done
eight years afterwards by the brothers Oscar and Richard Hertwig in
their careful and extensive comparative studies. In their masterly
Coelum Theory: An Attempt to Explain the Middle Germinal Layer [not
translated] (1881) they showed that in most of the metazoa, especially
in all the vertebrates, the body-cavity arises in the same way, by the
outgrowth of two sacs from the inner layer. These two coelom-pouches
proceed from the rudimentary mouth of the gastrula, between the two
primary layers. The inner plate of the two-layered coelom-pouch (the
visceral layer) joins itself to the entoderm; the outer plate
(parietal layer) unites with the ectoderm. Thus are formed the
double-layered gut-wall within and the double-layered body-wall
without; and between the two is formed the cavity of the coelom, by
the blending of the right and left coelom-sacs. We shall see this more
fully in Chapter 1.10.
The many new points of view and fresh ideas suggested by my gastraea
theory and Hertwig's coelom theory led to the publication of a number
of writings on the theory of germinal layers. Most of them set out to
oppose it at first, but in the end the majority supported it. Of late
years both theories are accepted in their essential features by nearly
every competent man of science, and light and order have been
introduced into this once dark and contradictory field of research. A
further cause of congratulation for this solution of the great
embryological controversy is that it brought with it a recognition of
the need for phylogenetic study and explanation.
Interest and practice in embryological research have been remarkably
stimulated during the past thirty years by this appreciation of
phylogenetic methods. Hundreds of assiduous and able observers are now
engaged in the development of comparative embryology and its
establishment on a basis of evolution, whereas they numbered only a
few dozen not many decades ago. It would take too long to enumerate
even the most important of the countless valuable works which have
enriched embryological literature since that time. References to them
will be found in the latest manuals of embryology of Kolliker,
Balfour, Hertwig, Kollman, Korschelt, and Heider.
Kolliker's Entwickelungsgeschichte des Menschen und der hoherer
Thiere, the first edition of which appeared forty-two years ago, had
the rare merit at that time of gathering into presentable form the
scattered attainments of the science, and expounding them in some sort
of unity on the basis of the cellular theory and the theory of
germinal layers. Unfortunately, the distinguished Wurtzburg anatomist,
to whom comparative anatomy, histology, and ontogeny owe so much, is
opposed to the theory of descent generally and to Darwinism in
particular. All the other manuals I have mentioned take a decided
stand on evolution. Francis Balfour has carefully collected and
presented with discrimination, in his Manual of Comparative Embryology
(1880), the very scattered and extensive literature of the subject; he
has also widened the basis of the gastraea theory by a comparative
description of the rise of the organs from the germinal layers in all
the chief groups of the animal kingdom, and has given a most thorough
empirical support to the principles I have formulated. A comparison of
his work with the excellent Text-book of the Embryology of the
Vertebrates (1890) [translation 1895] of Korschelt and Heider shows
what astonishing progress has been made in the science in the course
of ten years. I would especially recommend the manuals of Julius
Kollmann and Oscar Hertwig to those readers who are stimulated to
further study by these chapters on human embryology. Kollmann's work
is commendable for its clear treatment of the subject and very fine
original illustrations; its author adheres firmly to the biogenetic
law, and uses it throughout with considerable profit. That is not the
case in Oscar Hertwig's recent Text-book of the Embryology of Man and
the Mammals [translations 1892 and 1899] (seventh edition 1902). This
able anatomist has of late often been quoted as an opponent of the
biogenetic law, although he himself had demonstrated its great value
thirty years ago. His recent vacillation is partly due to the timidity
which our "exact" scientists have with regard to hypotheses; though it
is impossible to make any headway in the explanation of facts without
them. However, the purely descriptive part of embryology in Hertwig's
Text-book is very thorough and reliable.
A new branch of embryological research has been studied very
assiduously in the last decade of the nineteenth century--namely,
"experimental embryology." The great importance which has been
attached to the application of physical experiments to the living
organism for the last hundred years, and the valuable results that it
has given to physiology in the study of the vital phenomena, have led
to its extension to embryology. I was the first to make experiments of
this kind during a stay of four months on the Canary Island,
Lanzerote, in 1866. I there made a thorough investigation of the
almost unknown embryology of the siphonophorae. I cut a number of the
embryos of these animals (which develop freely in the water, and pass
through a very curious transformation), at an early stage, into
several pieces, and found that a fresh organism (more or less
complete, according to the size of the piece) was developed from each
particle. More recently some of my pupils have made similar
experiments with the embryos of vertebrates (especially the frog) and
some of the invertebrates. Wilhelm Roux, in particular, has made
extensive experiments, and based on them a special "mechanical
embryology," which has given rise to a good deal of discussion and
controversy. Roux has published a special journal for these subjects
since 1895, the Archiv fur Entwickelungsmechanik. The contributions to
it are very varied in value. Many of them are valuable papers on the
physiology and pathology of the embryo. Pathological experiments--the
placing of the embryo in abnormal conditions--have yielded many
interesting results; just as the physiology of the normal body has for
a long time derived assistance from the pathology of the diseased
organism. Other of these mechanical-embryological articles return to
the erroneous methods of His, and are only misleading. This must be
said of the many contributions of mechanical embryology which take up
a position of hostility to the theory of descent and its chief
embryological foundation--the biogenetic law. This law, however, when
rightly understood, is not opposed to, but is the best and most solid
support of, a sound mechanical embryology. Impartial reflection and a
due attention to paleontology and comparative anatomy should convince
these one-sided mechanicists that the facts they have discovered--and,
indeed, the whole embryological process--cannot be fully understood
without the theory of descent and the biogenetic law.