Copyright
Ernst Heinrich Philipp August Haeckel.

The Evolution of Man — Volume 1 online

. (page 25 of 26)
Online LibraryErnst Heinrich Philipp August HaeckelThe Evolution of Man — Volume 1 → online text (page 25 of 26)
Font size
QR-code for this ebook


cavity, and the face not yet shaped. The heart shows all its four
sections; it is very large, and almost fills the whole of the pectoral
cavity (Figure 1.183 ov). Behind it are the very small rudimentary
lungs. The primitive kidneys (m) are very large; they fill the greater
part of the abdominal cavity, and extend from the liver (f) to the
pelvic gut. Thus at the end of the first month all the chief organs
are already outlined. But there are at this stage no features by which
the human embryo materially differs from that of the dog, the hare,
the ox, or the horse - in a word, of any other higher mammal. All these
embryos have the same, or at least a very similar, form; they can at
the most be distinguished from the human embryo by the total size of
the body or some other insignificant difference in size. Thus, for
instance, in man the head is larger in proportion to the trunk than in
the ox. The tail is rather longer in the dog than in man. These are
all negligible differences. On the other hand, the whole internal
organisation and the form and arrangement of the various organs are
essentially the same in the human embryo of four weeks as in the
embryos of the other mammals at corresponding stages.

(FIGURE 1.191. Human embryo of sixteen to eighteen days. (From Coste.)
Magnified. The embryo is surrounded by the amnion, (a), and lies free
with this in the opened embryonic vesicle. The belly is drawn up by
the large yelk-sac (d), and fastened to the inner wall of the
embryonic membrane by the short and thick pedicle (b). Hence the
normal convex curve of the back (Figure 1.190) is here changed into an
abnormal concave surface. h heart, m parietal mesoderm. The spots on
the outer wall of the serolemma are the roots of the branching
chorion-villi, which are free at the border.

FIGURE 1.192. Human embryo of the fourth week, one-third of an inch
long, lying in the dissected chorion.

FIGURE 1.193. Human embryo of the fourth week, with its membranes,
like Figure 1.192, but a little older. The yelk-sac is rather smaller,
the amnion and chorion larger.)

It is otherwise in the second month of human development. Figure 1.179
represents a human embryo of six weeks (VI), one of seven weeks (VII),
and one of eight weeks (VIII), at natural size. The differences which
mark off the human embryo from that of the dog and the lower mammals
now begin to be more pronounced. We can see important differences at
the sixth, and still more at the eighth week, especially in the
formation of the head. The size of the various sections of the brain
is greater in man, and the tail is shorter. Other differences between
man and the lower mammals are found in the relative size of the
internal organs. But even at this stage the human embryo differs very
little from that of the nearest related mammals - the apes, especially
the anthropomorphic apes. The features by means of which we
distinguish between them are not clear until later on. Even at a much
more advanced stage of development, when we can distinguish the human
foetus from that of the ungulates at a glance, it still closely
resembles that of the higher apes. At last we get the distinctive
features, and we can distinguish the human embryo confidently at the
first glance from that of all other mammals during the last four
months of foetal life - from the sixth to the ninth month of pregnancy.
Then we begin to find also the differences between the various races
of men, especially in regard to the formation of the skull and the
face. (Cf. Chapter 2.23.)

(FIGURE 1.194. Human embryo with its membranes, six weeks old. The
outer envelope of the whole ovum is the chorion, thickly covered with
its branching villi, a product of the serous membrane. The embryo is
enclosed in the delicate amnion-sac. The yelk-sac is reduced to a
small pear-shaped umbilical vesicle; its thin pedicle, the long
vitelline duct, is enclosed in the umbilical cord. In the latter,
behind the vitelline duct, is the much shorter pedicle of the
allantois, the inner lamina of which (the gut-gland layer) forms a
large vesicle in most of the mammals, while the outer lamina is
attached to the inner wall of the outer embryonic coat, and forms the
placenta there. (Half diagrammatic.))

The striking resemblance that persists so long between the embryo of
man and of the higher apes disappears much earlier in the lower apes.
It naturally remains longest in the large anthropomorphic apes
(gorilla, chimpanzee, orang, and gibbon). The physiognomic similarity
of these animals, which we find so great in their earlier years,
lessens with the increase of age. On the other hand, it remains
throughout life in the remarkable long-nosed ape of Borneo (Nasalis
larvatus). Its finely-shaped nose would be regarded with envy by many
a man who has too little of that organ. If we compare the face of the
long-nosed ape with that of abnormally ape-like human beings (such as
the famous Miss Julia Pastrana, Figure 1.185), it will be admitted to
represent a higher stage of development. There are still people among
us who look especially to the face for the "image of God in man." The
long-nosed ape would have more claim to this than some of the
stumpy-nosed human individuals one meets.

This progressive divergence of the human from the animal form, which
is based on the law of the ontogenetic connection between related
forms, is found in the structure of the internal organs as well as in
external form. It is also expressed in the construction of the
envelopes and appendages that we find surrounding the foetus
externally, and that we will now consider more closely. Two of these
appendages - the amnion and the allantois - are only found in the three
higher classes of vertebrates, while the third, the yelk-sac, is found
in most of the vertebrates. This is a circumstance of great
importance, and it gives us valuable data for constructing man's
genealogical tree.

(FIGURE 1.195. Diagram of the embryonic organs of the mammal (foetal
membranes and appendages). (From Turner.) E, M, H outer, middle, and
inner germ layer of the embryonic shield, which is figured in median
longitudinal section, seen from the left. am amnion. AC amniotic
cavity, UV yelk-sac or umbilical vesicle, ALC allantois, al pericoelom
or serocoelom (inter-amniotic cavity), sz serolemma (or serous
membrane), pc prochorion (with villi).)

As regards the external membrane that encloses the ovum in the mammal
womb, we find it just the same in man as in the higher mammals. The
ovum is, the reader will remember, first surrounded by the transparent
structureless ovolemma or zona pellucida (Figures 1.1 and 1.14). But
very soon, even in the first week of development, this is replaced by
the permanent chorion. This is formed from the external layer of the
amnion, the serolemma, or "serous membrane," the formation of which we
shall consider presently; it surrounds the foetus and its appendages
as a broad, completely closed sac; the space between the two, filled
with clear watery fluid, is the serocoelom, or interamniotic cavity
("extra-embryonic body-cavity"). But the smooth surface of the sac is
quickly covered with numbers of tiny tufts, which are really hollow
outgrowths like the fingers of a glove (Figures 1.186, 1.191 and 1.198
chz). They ramify and push into the corresponding depressions that are
formed by the tubular glands of the mucous membrane of the maternal
womb. Thus, the ovum secures its permanent seat (Figures 1.186 to
1.194).

In human ova of eight to twelve days this external membrane, the
chorion, is already covered with small tufts or villi, and forms a
ball or spheroid of one-fourth to one-third of an inch in diameter
(Figures 1.186 to 1.188). As a large quantity of fluid gathers inside
it, the chorion expands more and more, so that the embryo only
occupies a small part of the space within the vesicle. The villi of
the chorion grow larger and more numerous. They branch out more and
more. At first the villi cover the whole surface, but they afterwards
disappear from the greater part of it; they then develop with
proportionately greater vigour at a spot where the placenta is formed
from the allantois.

When we open the chorion of a human embryo of three weeks, we find on
the ventral side of the foetus a large round sac, filled with fluid.
This is the yelk-sac, or "umbilical vesicle," the origin of which we
have considered previously. The larger the embryo becomes the smaller
we find the yelk-sac. In the end we find the remainder of it in the
shape of a small pear-shaped vesicle, fastened to a long thin stalk
(or pedicle), and hanging from the open belly of the foetus (Figure
1.194). This pedicle is the vitelline duct, and is separated from the
body at the closing of the navel.

Behind the yelk-sac a second appendage, of much greater importance, is
formed at an early stage at the belly of the mammal embryo. This is
the allantois or "primitive urinary sac," an important embryonic
organ, only found in the three higher classes of vertebrates. In all
the amniotes the allantois quickly appears at the hinder end of the
alimentary canal, growing out of the cavity of the pelvic gut (Figure
1.147 r, u, Figure 1.195 ALC}.

(FIGURE 1.196. Diagrammatic frontal section of the pregnant human
womb. (From Longet.) The embryo hangs by the umbilical cord, which
encloses the pedicle of the allantois (al). nb umbilical vessel, am
amnion, ch chorion, ds decidua serotina, dv decidua vera, dr decidua
reflexa, z villi of the placenta, c cervix uteri, u uterus.)

The further development of the allantois varies considerably in the
three sub-classes of the mammals. The two lower sub-classes,
monotremes and marsupials, retain the simpler structure of their
ancestors, the reptiles. The wall of the allantois and the enveloping
serolemma remains smooth and without villi, as in the birds. But in
the third sub-class of the mammals the serolemma forms, by
invagination at its outer surface, a number of hollow tufts or villi,
from which it takes the name of the chorion or mallochorion. The
gut-fibre layer of the allantois, richly supplied with branches of the
umbilical vessel, presses into these tufts of the primary chorion, and
forms the "secondary chorion." Its embryonic blood-vessels are closely
correlated to the contiguous maternal blood-vessels of the environing
womb, and thus is formed the important nutritive apparatus of the
embryo which we call the placenta.

The pedicle of the allantois, which connects the embryo with the
placenta and conducts the strong umbilical vessels from the former to
the latter, is covered by the amnion, and, with this amniotic sheath
and the pedicle of the yelk-sac, forms what is called the umbilical
cord (Figure 1.196 al). As the large and blood-filled vascular network
of the foetal allantois attaches itself closely to the mucous lining
of the maternal womb, and the partition between the blood-vessels of
mother and child becomes much thinner, we get that remarkable
nutritive apparatus of the foetal body which is characteristic of the
placentalia (or choriata). We shall return afterwards to the closer
consideration of this (cf. Chapter 2.23).

In the various orders of mammals the placenta undergoes many
modifications, and these are in part of great evolutionary importance
and useful in classification. There is only one of these that need be
specially mentioned - the important fact, established by Selenka in
1890, that the distinctive human placentation is confined to the
anthropoids. In this most advanced group of the mammals the allantois
is very small, soon loses its cavity, and then, in common with the
amnion, undergoes certain peculiar changes. The umbilical cord
develops in this case from what is called the "ventral pedicle." Until
very recently this was regarded as a structure peculiar to man. We now
know from Selenka that the much-discussed ventral pedicle is merely
the pedicle of the allantois, combined with the pedicle of the amnion
and the rudimentary pedicle of the yelk-sac. It has just the same
structure in the orang and gibbon (Figure 1.197) and very probably in
the chimpanzee and gorilla, as in man; it is, therefore, not a
DISPROOF, but a striking fresh proof, of the blood-relationship of man
and the anthropoid apes.

(FIGURE 1.197. Male embryo of the Siamang-gibbon (Hylobates siamanga)
of Sumatra, two-thirds natural size; to the left the dissected uterus,
of which only the dorsal half is given. The embryo has been taken out,
and the limbs folded together; it is still connected by the umbilical
cord with the centre of the circular placenta which is attached to the
inside of the womb. This embryo takes the head-position in the womb,
and this is normal in man also.)

We find only in the anthropoid apes - the gibbon and orang of Asia and
the chimpanzee and gorilla of Africa - the peculiar and elaborate
formation of the placenta that characterises man (Figure 1.198). In
this case there is at an early stage an intimate blending of the
chorion of the embryo and the part of the mucous lining of the womb to
which it attaches. The villi of the chorion with the blood-vessels
they contain grow so completely into the tissue of the uterus, which
is rich in blood, that it becomes impossible to separate them, and
they form together a sort of cake. This comes away as the "afterbirth"
at parturition; at the same time, the part of the mucous lining of the
womb that has united inseparably with the chorion is torn away; hence
it is called the decidua ("falling-away membrane"), and also the
"sieve-membrane," because it is perforated like a sieve. We find a
decidua of this kind in most of the higher placentals; but it is only
in man and the anthropoid apes that it divides into three parts - the
outer, inner, and placental decidua. The external or true decidua
(Figure 1.196 du, Figure 1.199 g) is the part of the mucous lining of
the womb that clothes the inner surface of the uterine cavity wherever
it is not connected with the placenta. The placental or spongy decidua
(placentalis or serotina, Figure 1.196 ds, Figure 1.199 d) is really
the placenta itself, or the maternal part of it (placenta
uterina) - namely, that part of the mucous lining of the womb which
unites intimately with the chorion-villi of the foetal placenta. The
internal or false decidua (interna or reflexa, Figure 1.196 dr, Figure
1.199 f) is that part of the mucous lining of the womb which encloses
the remaining surface of the ovum, the smooth chorion (chorion laeve),
in the shape of a special thin membrane. The origin of these three
different deciduous membranes, in regard to which quite erroneous
views (still retained in their names) formerly prevailed, is now quite
clear, The external decidua vera is the specially modified and
subsequently detachable superficial stratum of the original mucous
lining of the womb. The placental decidua serotina is that part of the
preceding which is completely transformed by the ingrowth of the
chorion-villi, and is used for constructing the placenta. The inner
decidua reflexa is formed by the rise of a circular fold of the mucous
lining (at the border of the decidua vera and serotina), which grows
over the foetus (like the anmnion) to the end.

The peculiar anatomic features that characterise the human foetal
membranes are found in just the same way in the higher apes. Until
recently it was thought that the human embryo was distinguished by its
peculiar construction of a solid allantois and a special ventral
pedicle, and that the umbilical cord developed from this in a
different way than in the other mammals. The opponents of the
unwelcome "ape-theory" laid great stress on this, and thought they had
at last discovered an important indication that separated man from all
the other placentals. But the remarkable discoveries published by the
distinguished zoologist Selenka in 1890 proved that man shares these
peculiarities of placentation with the anthropoid apes, though they
are not found in the other apes. Thus the very feature which was
advanced by our critics as a disproof became a most important piece of
evidence in favour of our pithecoid origin.)

(FIGURE 1.198. Frontal section of the pregnant human womb, showing:
end of the decidua, uterine cavity, chorion (laeve), amniotic cavity,
foetal placenta, oviduct, spongy decidua serotina, umbilical vesicle,
amnion, decidua reflexa, decidua vera, muscular wall of the uterus,
mouth of the uterus. (From Turner.) The embryo (a month old) hangs in
the middle of the amniotic cavity by the ventral pedicle or umbilical
cord, which connects it with the placenta (above).

FIGURE 1.199. Human foetus, twelve weeks old, with its membranes.
Natural size. The umbilical cord goes from its navel to the placenta.
b amnion, c chorion, d placenta, d apostrophe, relics of villi on
smooth chorion, f internal or reflex decidua, g external or true
decidua. (From B. Schultze.)

FIGURE 1.200. Mature human foetus (at the end of pregnancy, in its
natural position, taken out of the uterine cavity). On the inner
surface of the latter (to the left) is the placenta, which is
connected by the umbilical cord with the child's navel. (From Bernhard
Schultze.))

Of the three vesicular appendages of the amniote embryo which we have
now described the amnion has no blood-vessels at any moment of its
existence. But the other two vesicles, the yelk-sac and the allantois,
are equipped with large blood-vessels, and these effect the
nourishment of the embryonic body. We may take the opportunity to make
a few general observations on the first circulation in the embryo and
its central organ, the heart. The first blood-vessels, the heart, and
the first blood itself, are formed from the gut-fibre layer. Hence it
was called by earlier embryologists the "vascular layer." In a sense
the term is quite correct. But it must not be understood as if all the
blood-vessels in the body came from this layer, or as if the whole of
this layer were taken up only with the formation of blood-vessels.
Neither of these suppositions is true. Blood-vessels may be formed
independently in other parts, especially in the various products of
the skin-fibre layer.

The first blood-vessels of the mammal embryo have been considered by
us previously, and we shall study the development of the heart in the
second volume.

(FIGURE 1.201. Vitelline vessels in the germinative area of a
chick-embryo, at the close of the third day of incubation. (From
Balfour.) The detached germinative area is seen from the ventral side:
the arteries are dark, the veins light. H heart, AA aorta-arches, Ao
aorta, R.Of.A right omphalo-mesenteric artery, S.T sinus terminalis,
L.Of and R.Of right and left omphalo-mesenteric veins, S.V sinus
venosus, D.C ductus Cuvieri, S.Ca.V and V.Ca fore and hind cardinal
veins.)

In every vertebrate it lies at first in the ventral wall of the
fore-gut, or in the ventral (or cardiac) mesentery, by which it is
connected for a time with the wall of the body. But it soon severs
itself from the place of its origin, and lies freely in a cavity - the
cardiac cavity. For a short time it is still connected with the former
by the thin plate of the mesocardium. Afterwards it lies quite free in
the cardiac cavity, and is only directly connected with the gut-wall
by the vessels which issue from it.

The fore-end of the spindle-shaped tube, which soon bends into an
S-shape (Figure 1.202), divides into a right and left branch. These
tubes are bent upwards arch-wise, and represent the first arches of
the aorta. They rise in the wall of the fore-gut, which they enclose
in a sense, and then unite above, in the upper wall of the fore
gut-cavity, to form a large single artery, that runs backward
immediately under the chorda, and is called the aorta (Figure 1.201
Ao). The first pair of aorta-arches rise on the inner wall of the
first pair of gill-arches, and so lie between the first gill-arch (k)
and the fore-gut (d), just as we find them throughout life in the
fishes. The single aorta, which results from the conjunction of these
two first vascular arches, divides again immediately into two parallel
branches, which run backwards on either side of the chorda. These are
the primitive aortas which we have already mentioned; they are also
called the posterior vertebral arteries. These two arteries now give
off at each side, behind, at right angles, four or five branches, and
these pass from the embryonic body to the germinative area, they are
called omphalo-mesenteric or vitelline arteries. They represent the
first beginning of a foetal circulation. Thus, the first blood-vessels
pass over the embryonic body and reach as far as the edge of the
germinative area. At first they are confined to the dark or "vascular"
area. But they afterwards extend over the whole surface of the
embryonic vesicle. In the end, the whole of the yelk-sac is covered
with a vascular net-work. These vessels have to gather food from the
contents of the yelk-sac and convey it to the embryonic body. This is
done by the veins, which pass first from the germinative area, and
afterwards from the yelk-sac, to the farther end of the heart. They
are called vitelline, or, frequently, omphalo-mesenteric, veins.

These vessels naturally atrophy with the degeneration of the umbilical
vesicle, and the vitelline circulation is replaced by a second, that
of the allantois. Large blood-vessels are developed in the wall of the
urinary sac or the allantois, as before, from the gut-fibre layer.
These vessels grow larger and larger, and are very closely connected
with the vessels that develop in the body of the embryo itself. Thus,
the secondary, allantoic circulation gradually takes the place of the
original vitelline circulation. When the allantois has attached itself
to the inner wall of the chorion and been converted into the placenta,
its blood-vessels alone effect the nourishment of the embryo. They are
called umbilical vessels, and are originally double - a pair of
umbilical arteries and a pair of umbilical veins. The two umbilical
veins (Figure 1.183 u), which convey blood from the placenta to the
heart, open it first into the united vitelline veins. The latter then
disappear, and the right umbilical vein goes with them, so that
henceforth a single large vein, the left umbilical vein, conducts all
the blood from the placenta to the heart of the embryo. The two
arteries of the allantois, or the umbilical arteries (Figures 1.183 n
and 1.184 n), are merely the ultimate terminations of the primitive
aortas, which are strongly developed afterwards. This umbilical
circulation is retained until the nine months of embryonic life are
over, and the human embryo enters into the world as the independent
individual. The umbilical cord (Figure 1.196 al), in which these large
blood-vessels pass from the embryo to the placenta, comes away,
together with the latter, in the after-birth, and with the use of the
lungs begins an entirely new form of circulation, which is confined to
the body of the infant.

(FIGURE 1.202. Boat-shaped embryo of the dog, from the ventral side,
magnified about ten times. In front under the forehead we can see the
first pair of gill-arches; underneath is the S-shaped heart, at the
sides of which are the auditory vesicles. The heart divides behind
into the two vitelline veins, which expand in the germinative area
(which is torn off all round). On the floor of the open belly lie,
between the protovertebrae, the primitive aortas, from which five
pairs of vitelline arteries are given off. (From Bischoff.))

There is a great phylogenetic significance in the perfect agreement
which we find between man and the anthropoid apes in these important
features of embryonic circulation, and the special construction of the
placenta and the umbilical cord. We must infer from it a close
blood-relationship of man and the anthropomorphic apes - a common
descent of them from one and the same extinct group of lower apes.
Huxley's "pithecometra-principle" applies to these ontogenetic
features as much as to any other morphological relations: "The
differences in construction of any part of the body are less between
man and the anthropoid apes than between the latter and the lower
apes."

This important Huxleian law, the chief consequence of which is "the
descent of man from the ape," has lately been confirmed in an
interesting and unexpected way from the side of the experimental
physiology of the blood. The experiments of Hans Friedenthal at Berlin
have shown that human blood, mixed with the blood of lower apes, has a
poisonous effect on the latter; the serum of the one destroys the


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 25

Online LibraryErnst Heinrich Philipp August HaeckelThe Evolution of Man — Volume 1 → online text (page 25 of 26)