Ernst Heinrich Philipp August Haeckel.

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the branchial arches and the clefts that alternate with them is four
or five on each side in the higher vertebrates (Figure 1.170 d, f, f
apostrophe, f double apostrophe). In some of the fishes (selachii) and
in the cyclostoma we find six or seven of them permanently.

These remarkable structures had originally the function of respiratory
organs - gills. In the fishes the water that serves for breathing, and
is taken in at the mouth, still always passes out by the branchial
clefts at the sides of the gullet. In the higher vertebrates they
afterwards disappear. The branchial arches are converted partly into
the jaws, partly into the bones of the tongue and the ear. From the
first gill-cleft is formed the tympanic cavity of the ear.

There are few parts of the vertebrate organism that, like the outer
covering or integument of the body, are not subject to metamerism. The
outer skin (epidermis) is unsegmented from the first, and proceeds
from the continuous horny plate. Moreover, the underlying cutis is
also not metamerous, although it develops from the segmental structure
of the cutis-plates (Figures 1.161 and 1.162 cp). The vertebrates are
strikingly and profoundly different from the articulates in these
respects also.

Further, most of the vertebrates still have a number of unarticulated
organs, which have arisen locally, by adaptation of particular parts
of the body to certain special functions. Of this character are the
sense-organs in the episoma, and the limbs, the heart, the spleen, and
the large visceral glands - lungs, liver, pancreas, etc. - in the
hyposoma. The heart is originally only a local spindle-shaped
enlargement of the large ventral blood-vessel or principal vein, at
the point where the subintestinal passes into the branchial artery, at
the limit of the head and trunk (Figures 1.170 and 1.171). The three
higher sense-organs - nose, eye, and ear - were originally developed in
the same form in all the craniotes, as three pairs of small
depressions in the skin at the side of the head.

The organ of smell, the nose, has the appearance of a pair of small
pits above the mouth-aperture, in front of the head (Figure 1.169 n).
The organ of sight, the eye, is found at the side of the head, also in
the shape of a depression (Figures 1.169 l and 1.170 b), to which
corresponds a large outgrowth of the foremost cerebral vesicle on each
side. Farther behind, at each side of the head, there is a third
depression, the first trace of the organ of hearing (Figure 1.169 g).
As yet we can see nothing of the later elaborate structure of these
organs, nor of the characteristic build of the face.

(FIGURE 1.174. Development of the lizard's legs (Lacerta agilis), with
special relation to their blood-vessels. 1, 3, 5, 7, 9, 11 right
fore-leg; 13, 15 left fore-leg; 2, 4, 6, 8, 10, 12 right hind-leg; 14,
16 left hind-leg; SRV lateral veins of the trunk, VU umbilical vein.
(From F. Hochstetter.))

When the human embryo has reached this stage of development, it can
still scarcely be distinguished from that of any other higher
vertebrate. All the chief parts of the body are now laid down: the
head with the primitive skull, the rudiments of the three higher
sense-organs and the five cerebral vesicles, and the gill-arches and
clefts; the trunk with the spinal cord, the rudiment of the vertebral
column, the chain of metamera, the heart and chief blood-vessels, and
the kidneys. At this stage man is a higher vertebrate, but shows no
essential morphological difference from the embryos of the mammals,
the birds, the reptiles, etc. This is an ontogenetic fact of the
utmost significance. From it we can gather the most important
phylogenetic conclusions.

There is still no trace of the limbs. Although head and trunk are
separated and all the principal internal organs are laid down, there
is no indication whatever of the "extremities" at this stage; they are
formed later on. Here again we have a fact of the utmost interest. It
proves that the older vertebrates had no feet, as we find to be the
case in the lowest living vertebrates (amphioxus and the cyclostoma).
The descendants of these ancient footless vertebrates only acquired
extremities - two fore-legs and two hind-legs - at a much later stage of
development. These were at first all alike, though they afterwards
vary considerably in structure - becoming fins (of breast and belly) in
the fishes, wings and legs in the birds, fore and hind legs in the
creeping animals, arms and legs in the apes and man. All these parts
develop from the same simple original structure, which forms
secondarily from the trunk-wall (Figures 1.172 and 1.173). They have
always the appearance of two pairs of small buds, which represent at
first simple roundish knobs or plates. Gradually each of these plates
becomes a large projection, in which we can distinguish a small inner
part and a broader outer part. The latter is the rudiment of the foot
or hand, the former that of the leg or arm. The similarity of the
original rudiment of the limbs in different groups of vertebrates is
very striking.

(FIGURE 1.175. Human embryo, five weeks old, half an inch long, seen
from the right, magnified ten times. (From Russel Bardeen and Harmon
Lewis.) In the undissected head we see the eye, mouth, and ear. In the
trunk the skin and part of the muscles have been removed, so that the
cartilaginous vertebral column is free; the dorsal root of a spinal
nerve goes out from each vertebra (towards the skin of the back). In
the middle of the lower half of the figure part of the ribs and
intercostal muscles are visible. The skin and muscles have also been
removed from the right limbs; the internal rudiments of the five
fingers of the hand, and five toes of the foot, are clearly seen
within the fin-shaped plate, and also the strong network of nerves
that goes from the spinal cord to the extremities. The tail projects
under the foot, and to the right of it is the first part of the
umbilical cord.)

How the five fingers or toes with their blood-vessels gradually
differentiate within the simple fin-like structure of the limbs can be
seen in the instance of the lizard in Figure 1.174. They are formed in
just the same way in man: in the human embryo of five weeks the five
fingers can clearly be distinguished within the fin-plate (Figure

The careful study and comparison of human embryos with those of other
vertebrates at this stage of development is very instructive, and
reveals more mysteries to the impartial student than all the religions
in the world put together. For instance, if we compare attentively the
three successive stages of development that are represented, in twenty
different amniotes we find a remarkable likeness. When we see that as
a fact twenty different amniotes of such divergent characters develop
from the same embryonic form, we can easily understand that they may
all descend from a common ancestor.

(FIGURES 1.176 TO 1.178. Embryos of the bat (Vespertilio murinus) at
three different stages. (From Oscar Schultze.) Figure 1.176:
Rudimentary limbs (v fore-leg, h hind-leg). l lenticular depression, r
olfactory pit, ok upper jaw, uk lower jaw, k2, k3, k4 first, second
and third gill-arches, a amnion, n umbilical vessel, d yelk-sac.
Figure 1.177: Rudiment of flying membrane, membranous fold between
fore and hind leg. n umbilical vessel, o ear-opening, f flying
membrane. Figure 1.178: The flying membrane developed and stretched
across the fingers of the hands, which cover the face.)

In the first stage of development, in which the head with the five
cerebral vesicles is already clearly indicated, but there are no
limbs, the embryos of all the vertebrates, from the fish to man, are
only incidentally or not at all different from each other. In the
second stage, which shows the limbs, we begin to see differences
between the embryos of the lower and higher vertebrates; but the human
embryo is still hardly distinguishable from that of the higher
mammals. In the third stage, in which the gill-arches have disappeared
and the face is formed, the differences become more pronounced. These
are facts of a significance that cannot be exaggerated.* (* Because
they show how the most diverse structures may be developed from a
common form. As we actually see this in the case of the embryos, we
have a right to assume it in that of the stem-forms. Nevertheless,
this resemblance, however great, is never a real identity. Even the
embryos of the different individuals of one species are usually not
really identical. If the reader can consult the complete edition of
this work at a library, he will find six plates illustrating these
twenty embryos.)

If there is an intimate causal connection between the processes of
embryology and stem-history, as we must assume in virtue of the laws
of heredity, several important phylogenetic conclusions follow at once
from these ontogenetic facts. The profound and remarkable similarity
in the embryonic development of man and the other vertebrates can only
be explained when we admit their descent from a common ancestor. As a
fact, this common descent is now accepted by all competent scientists;
they have substituted the natural evolution for the supernatural
creation of organisms.


Among the many interesting phenomena that we have encountered in the
course of human embryology, there is an especial importance in the
fact that the development of the human body follows from the beginning
just the same lines as that of the other viviparous mammals. As a
fact, all the embryonic peculiarities that distinguish the mammals
from other animals are found also in man; even the ovum with its
distinctive membrane (zona pellucida, Figure 1.14) shows the same
typical structure in all mammals (apart from the older oviparous
monotremes). It has long since been deduced from the structure of the
developed man that his natural place in the animal kingdom is among
the mammals. Linne (1735) placed him in this class with the apes, in
one and the same order (primates), in his Systema Naturae. This
position is fully confirmed by comparative embryology. We see that man
entirely resembles the higher mammals, and most of all the apes, in
embryonic development as well as in anatomic structure. And if we seek
to understand this ontogenetic agreement in the light of the
biogenetic law, we find that it proves clearly and necessarily the
descent of man from a series of other mammals, and proximately from
the primates. The common origin of man and the other mammals from a
single ancient stem-form can no longer be questioned; nor can the
immediate blood-relationship of man and the ape.

(FIGURE 1.179. Human embryos from the second to the fifteenth week,
natural size, seen from the left, the curved back turned towards the
right. (Mostly from Ecker.) II of fourteen days. III of three weeks.
IV of four weeks. V of five weeks. VI of six weeks. VII of seven
weeks. VIII of eight weeks. XII of twelve weeks. XV of fifteen weeks.)

The essential agreement in the whole bodily form and inner structure
is still visible in the embryo of man and the other mammals at the
late stage of development at which the mammal-body can be recognised
as such. But at a somewhat earlier stage, in which the limbs,
gill-arches, sense-organs, etc., are already outlined, we cannot yet
recognise the mammal embryos as such, or distinguish them from those
of birds and reptiles. When we consider still earlier stages of
development, we are unable to discover any essential difference in
bodily structure between the embryos of these higher vertebrates and
those of the lower, the amphibia and fishes. If, in fine, we go back
to the construction of the body out of the four germinal layers, we
are astonished to perceive that these four layers are the same in all
vertebrates, and everywhere take a similar part in the building-up of
the fundamental organs of the body. If we inquire as to the origin of
these four secondary layers, we learn that they always arise in the
same way from the two primary layers; and the latter have the same
significance in all the metazoa (i.e., all animals except the
unicellulars). Finally, we see that the cells which make up the
primary germinal layers owe their origin in every case to the repeated
cleavage of a single simple cell, the stem-cell or fertilised ovum.

(FIGURE 1.180. Very young human embryo of the fourth week, one-fourth
of an inch long (taken from the womb of a suicide eight hours after
death). (From Rabl.) n nasal pits, a eye, u lower jaw, z arch of hyoid
bone, k3 and k4 third and fourth gill-arch, h heart; s primitive
segments, vg fore-limb (arm), hg hind-limb (leg), between the two the
ventral pedicle.)

It is impossible to lay too much stress on this remarkable agreement
in the chief embryonic features in man and the other animals. We shall
make use of it later on for our monophyletic theory of descent - the
hypothesis of a common descent of man and all the metazoa from the
gastraea. The first rudiments of the principal parts of the body,
especially the oldest organ, the alimentary canal, are the same
everywhere; they have always the same extremely simple form. All the
peculiarities that distinguish the various groups of animals from each
other only appear gradually in the course of embryonic development;
and the closer the relation of the various groups, the later they are
found. We may formulate this phenomenon in a definite law, which may
in a sense be regarded as an appendix to our biogenetic law. This is
the law of the ontogenetic connection of related animal forms. It
runs: The closer the relation of two fully-developed animals in
respect of their whole bodily structure, and the nearer they are
connected in the classification of the animal kingdom, the longer do
their embryonic forms retain their identity, and the longer is it
impossible (or only possible on the ground of subordinate features) to
distinguish between their embryos. This law applies to all animals
whose embryonic development is, in the main, an hereditary summary of
their ancestral history, or in which the original form of development
has been faithfully preserved by heredity. When, on the other hand, it
has been altered by cenogenesis, or disturbance of development, we
find a limitation of the law, which increases in proportion to the
introduction of new features by adaptation (cf. Chapter 1.1). Thus the
apparent exceptions to the law can always be traced to cenogenesis.

(FIGURE 1.181. Human embryo of the middle of the fifth week, one-third
of an inch long. (From Rabl.) Letters as in Figure 1.180, except sk
curve of skull, ok upper jaw, hb neck-indentation.)

When we apply to man this law of the ontogenetic connection of related
forms, and run rapidly over the earliest stages of human development
with an eye to it, we notice first of all the structural identity of
the ovum in man and the other mammals at the very beginning (Figures
1.1 and 1.14). The human ovum possesses all the distinctive features
of the ovum of the viviparous mammals, especially the characteristic
formation of its membrane (zona pellucida), which clearly
distinguishes it from the ovum of all other animals. When the human
foetus has attained the age of fourteen days, it forms a round vesicle
(or "embryonic vesicle") about a quarter of an inch in diameter. A
thicker part of its border forms a simple sole-shaped embryonic shield
one-twelfth of an inch long (Figure 1.133). On its dorsal side we find
in the middle line the straight medullary furrow, bordered by the two
parallel dorsal or medullary swellings. Behind, it passes by the
neurenteric canal into the primitive gut or primitive groove. From
this the folding of the two coelom-pouches proceeds in the same way as
in the other mammals (cf. Figures 1.96 and 1.97). In the middle of the
sole-shaped embryonic shield the first primitive segments immediately
begin to make their appearance. At this age the human embryo cannot be
distinguished from that of other mammals, such as the hare or dog.

A week later (or after the twenty-first day) the human embryo has
doubled its length; it is now about one-fifth of an inch long, and,
when seen from the side, shows the characteristic bend of the back,
the swelling of the head-end, the first outline of the three higher
sense-organs, and the rudiments of the gill-clefts, which pierce the
sides of the neck (Figure 1.179, III). The allantois has grown out of
the gut behind. The embryo is already entirely enclosed in the amnion,
and is only connected in the middle of the belly by the vitelline duct
with the embryonic vesicle, which changes into the yelk-sac. There are
no extremities or limbs at this stage, no trace of arms or legs. The
head-end has been strongly differentiated from the tail-end; and the
first outlines of the cerebral vesicles in front, and the heart below,
under the fore-arm, are already more or less clearly seen. There is as
yet no real face. Moreover, we seek in vain at this stage a special
character that may distinguish the human embryo from that of other

(FIGURE 1.182. Median longitudinal section of the tail of a human
embryo, two-thirds of an inch long. (From Ross Granville Harrison.)
Med medullary tube, Ca.fil caudal filament, ch chorda, ao caudal
artery, V.c.i caudal vein, an anus, sinus urogenitalis.)

A week later (after the fourth week, on the twenty-eighth to thirtieth
day of development) the human embryo has reached a length of about
one-third of an inch (Figure 1.179 IV). We can now clearly distinguish
the head with its various parts; inside it the five primitive cerebral
vesicles (fore-brain, middle-brain, intermediate-brain, hind-brain,
and after-brain); under the head the gill-arches, which divide the
gill-clefts; at the sides of the head the rudiments of the eyes, a
couple of pits in the outer skin, with a pair of corresponding simple
vesicles growing out of the lateral wall of the fore-brain (Figures
1.180, 1.181 a). Far behind the eyes, over the last gill-arches, we
see a vesicular rudiment of the auscultory organ. The rudimentary
limbs are now clearly outlined - four simple buds of the shape of round
plates, a pair of fore (vg) and a pair of hind legs (hg), the former a
little larger than the latter. The large head bends over the trunk,
almost at a right angle. The latter is still connected in the middle
of its ventral side with the embryonic vesicle; but the embryo has
still further severed itself from it, so that it already hangs out as
the yelk-sac. The hind part of the body is also very much curved, so
that the pointed tail-end is directed towards the head. The head and
face-part are sunk entirely on the still open breast. The bend soon
increases so much that the tail almost touches the forehead (Figure
1.179 V.; Figure 1.181). We may then distinguish three or four special
curves on the round dorsal surface - namely, a skull-curve in the
region of the second cerebral vesicle, a neck-curve at the beginning
of the spinal cord, and a tail-curve at the fore-end. This pronounced
curve is only shared by man and the higher classes of vertebrates (the
amniotes); it is much slighter, or not found at all, in the lower
vertebrates. At this age (four weeks) man has a considerable tail,
twice as long as his legs. A vertical longitudinal section through the
middle plane of this tail (Figure 1.182) shows that the hinder end of
the spinal marrow extends to the point of the tail, as also does the
underlying chorda (ch), the terminal continuation of the vertebral
column. Of the latter, the rudiments of the seven coccygeal (or
lowest) vertebrae are visible - thirty-two indicates the third and
thirty-six the seventh of these. Under the vertebral column we see the
hindmost ends of the two large blood-vessels of the tail, the
principal artery (aorta caudalis or arteria sacralis media, Ao), and
the principal vein (vena caudalis or sacralis media). Underneath is
the opening of the anus (an) and the urogenital sinus ( From
this anatomic structure of the human tail it is perfectly clear that
it is the rudiment of an ape-tail, the last hereditary relic of a long
hairy tail, which has been handed down from our tertiary primate
ancestors to the present day.

(FIGURE 1.183. Human embryo, four weeks old, opened on the ventral
side. Ventral and dorsal walls are cut away, so as to show the
contents of the pectoral and abdominal cavities. All the appendages
are also removed (amnion, allantois, yelk-sac), and the middle part of
the gut. n eye, 3 nose, 4 upper jaw, 5 lower jaw, 6 second, 6 double
apostrophe, third gill-arch, ov heart (o right, o apostrophe, left
auricle; v right, v apostrophe, left ventricle), b origin of the
aorta, f liver (u umbilical vein), e gut (with vitelline artery, cut
off at a apostrophe), j apostrophe, vitelline vein, m primitive
kidneys, t rudimentary sexual glands, r terminal gut (cut off at the
mesentery z), n umbilical artery, u umbilical vein, 9 fore-leg, 9
apostrophe, hind-leg. (From Coste.)

FIGURE 1.184. Human embryo, five weeks old, opened from the ventral
side (as in Figure 1.183). Breast and belly-wall and liver are
removed. 3 outer nasal process, 4 upper jaw, 5 lower jaw, z tongue, v
right, v apostrophe, left ventricle of heart, o apostrophe, left
auricle, b origin of aorta, b apostrophe, b double apostrophe, b
triple apostrophe, first, second, and third aorta-arches, c, c
apostrophe, c double apostrophe, vena cava, ae lungs (y pulmonary
artery), e stomach, m primitive kidneys (j left vitelline vein, s
cystic vein, a right vitelline artery, n umbilical artery, u umbilical
vein), x vitelline duct, i rectum, 8 tail, 9 fore-leg, 9 apostrophe,
hind-leg. (From Coste.))

It sometimes happens that we find even external relics of this tail
growing. According to the illustrated works of Surgeon-General
Bernhard Ornstein, of Greece, these tailed men are not uncommon; it is
not impossible that they gave rise to the ancient fables of the
satyrs. A great number of such cases are given by Max Bartels in his
essay on "Tailed Men" (1884, in the Archiv fur Anthropologie, Band
15), and critically examined. These atavistic human tails are often
mobile; sometimes they contain only muscles and fat, sometimes also
rudiments of caudal vertebrae. They have a length of eight to ten
inches and more. Granville Harrison has very carefully studied one of
these cases of "pigtail," which he removed by operation from a six
months old child in 1901. The tail moved briskly when the child cried
or was excited, and was drawn up when at rest.

(FIGURE 1.185. The head of Miss Julia Pastrana. (From a photograph by

FIGURE 1.186. Human ovum of twelve to thirteen days (?). (From Allen
Thomson.) 1. Not opened, natural size. 2. Opened and magnified. Within
the outer chorion the tiny curved foetus lies on the large embryonic
vesicle, to the left above.

FIGURE 1.187. Human ovum of ten days. (From Allen Thomson.) Natural
size, opened; the small foetus in the right half, above.

FIGURE 1.188. Human foetus of ten days, taken from the preceding ovum,
magnified ten times, a yelk-sac, b neck (the medullary groove already
closed), c head (with open medullary groove), d hind part (with open
medullary groove), e a shred of the amnion.

FIGURE 1.189. Human ovum of twenty to twenty-two days. (From Allen
Thomson.) Natural size, opened. The chorion forms a spacious vesicle,
to the inner wall of which the small foetus (to the right above) is
attached by a short umbilical cord.

FIGURE 1.190. Human foetus of twenty to twenty-two days, taken from
the preceding ovum, magnified. a amnion, b yelk-sac, c lower-jaw
process of the first gill-arch, d upper-jaw process of same, e second
gill-arch (two smaller ones behind). Three gill-clefts are clearly
seen. f rudimentary fore-leg, g auditory vesicle, h eye, i heart.)

In the opinion of some travellers and anthropologists, the atavistic
tail-formation is hereditary in certain isolated tribes (especially in
south-eastern Asia and the archipelago), so that we might speak of a
special race or "species" of tailed men (Homo caudatus). Bartels has
"no doubt that these tailed men will be discovered in the advance of
our geographical and ethnographical knowledge of the lands in
question" (Archiv fur Anthropologie, Band 15 page 129).

When we open a human embryo of one month (Figure 1.183), we find the
alimentary canal formed in the body-cavity, and for the most part cut
off from the embryonic vesicle. There are both mouth and anus
apertures. But the mouth-cavity is not yet separated from the nasal

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