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about the embryo, and join above it, they come at last to form a
spacious sac-like membrane about it. This envelope takes the name of
the germinative membrane, or water-membrane, or amnion (Figure 1.142
am). The embryo floats in a watery fluid, which fills the space
between the embryo and the amnion, and is called the amniotic fluid
(Figures 1.141 and 1.142 ah). We will deal with this remarkable
formation and with the allantois later on (Chapter 1.15). In front of
the allantois the yelk-sac or umbilical vesicle (ds), the remainder of
the original embryonic vesicle, starts from the open belly of the
embryo (Figure 1.138 kh). In more advanced embryos, in which the
gastric wall and the ventral wall are nearly closed, it hangs out of
the navel-opening in the shape of a small vesicle with a stalk
(Figures 1.141 and 1.142 ds). The more the embryo grows, the smaller
becomes the vitelline (yelk) sac. At first the embryo looks like a
small appendage of the large embryonic vesicle. Afterwards it is the
yelk-sac, or the remainder of the embryonic vesicle, that seems a
small pouch-like appendage of the embryo (Figure 1.142 ds). It ceases
to have any significance in the end. The very wide opening, through
which the gastric cavity at first communicates with the umbilical
vesicle, becomes narrower and narrower, and at last disappears
altogether. The navel, the small pit-like depression that we find in
the developed man in the middle of the abdominal wall, is the spot at
which the remainder of the embryonic vesicle (the umbilical vesicle)
originally entered into the ventral cavity, and joined on to the
growing gut.

(FIGURES 1.138 TO 1.142. Five diagrammatic longitudinal sections of
the maturing mammal embryo and its envelopes. In Figures 1.138 to
1.141 the longitudinal section passes through the sagittal or middle
plane of the body, dividing the right and left halves; in Figure 1.142
the embryo is seen from the left side. In Figure 1.138 the tufted it
prochorion (dd apostrophe) encloses the germinal vesicle, the wall of
which consists of the two primary layers. Between the outer (a) and
inner (i) layer the middle layer (m) has been developed in the region
of the germinative area. In Figure 1.139 the embryo (e) begins to
separate from the embryonic vesicle (ds), while the wall of the
amnion-fold rises about it (in front as head-sheath, ks, behind as
tail-sheath, ss). In Figure 1.140 the edges of the amniotic fold (am)
rise together over the back of the embryo, and form the amniotic
cavity (ah); as the embryo separates more completely from the
embryonic vesicle (ds) the alimentary canal (dd) is formed, from the
hinder end of which the allantois grows (al). In Figure 1.141 the
allantois is larger; the yelk-sac (ds) smaller. In Figure 1.142 the
embryo shows the gill-clefts and the outline of the two legs; the
chorion has formed branching villi (tufts.) In all four figures e =
embryo, a outer germinal layer, m middle germinal layer, i inner
germinal layer, am amnion (ks head-sheath, ss tail-sheath), ah
amniotic cavity, as amniotic sheath of the umbilical cord, kh
embryonic vesicle, ds yelk-sac (umbilical vesicle), dg vitelline duct,
df gut-fibre layer, dd gut-gland layer, al allantois, vl = hh place of
heart, d vitelline membrane (ovolemma or prochorion), d apostrophe
tufts or villi of same, sh serous membrane (serolemma), sz tufts of
same, ch chorion, chz tufts or villi, st terminal vein, r pericoelom
or serocoelom (the space, filled with fluid, between the amnion and
chorion). (From Kolliker.))

The origin of the navel coincides with the complete closing of the
external ventral wall. In the amniotes the ventral wall originates in
the same way as the dorsal wall. Both are formed substantially from
the skin-fibre layer, and externally covered with the horn-plate, the
border section of the skin-sense layer. Both come into existence by
the conversion of the four flat germinal layers of the embryonic
shield into a double tube by folding from opposite directions; above,
at the back, we have the vertebral canal which encloses the medullary
tube, and below, at the belly, the wall of the body-cavity which
contains the alimentary canal (Figure 1.137).

We will consider the formation of the dorsal wall first, and that of
the ventral wall afterwards (Figures 1.143 to 1.147). In the middle of
the dorsal surface of the embryo there is originally, as we already
know, the medullary (mr) tube directly underneath the horn-plate (h),
from the middle part of which it has been developed. Later, however,
the provertebral plates (uw) grow over from the right and left between
these originally connected parts (Figures 1.145 and 1.146). The upper
and inner edges of the two provertebral plates push between the
horn-plate and medullary tube, force them away from each other, and
finally join between them in a seam that corresponds to the middle
line of the back. The coalescence of these two dorsal plates and the
closing in the middle of the dorsal wall take place in the same way as
the medullary tube, which is henceforth enclosed by the vertebral
tube. Thus is formed the dorsal wall, and the medullary tube takes up
a position inside the body. In the same way the provertebral mass
grows afterwards round the chorda, and forms the vertebral column.
Below this the inner and outer edge of the provertebral plate splits
on each side into two horizontal plates, of which the upper pushes
between the chorda and medullary tube, and the lower between the
chorda and gastric tube. As the plates meet from both sides above and
below the chorda, they completely enclose it, and so form the tubular,
outer chord-sheath, the sheath from which the vertebral column is
formed (perichorda, Figure 1.137 C, s; Figures 1.145 uwh, 1.146).

(FIGURES 1.143 TO 1.146. Transverse sections of embryos (of chicks).
Figure 1.143 of the second, Figure 1.144 of the third, Figure 1.145 of
the fourth, and Figure 1.146 of the fifth day of incubation. Figures
1.143 to 1.145 from Kolliker, magnified about 100 times; Figure 1.146
from Remak, magnified about twenty times. h horn-plate, mr medullary
tube, ung prorenal duct, un prorenal vesicles, hp skin-fibre layer, m
= mu = mp muscle-plate, uw provertebral plate (wh cutaneous rudiment
of the body of the vertebra, wb of the arch of the vertebra, wq the
rib or transverse continuation), uwh provertebral cavity, ch axial rod
or chorda, sh chorda-sheath, bh ventral wall, g hind and v fore root
of the spinal nerves, a = af = am amniotic fold, p body-cavity or
coeloma, df gut-fibre layer, ao primitive aortas, sa secondary aorta,
vc cardinal veins, d = dd gut-gland layer, dr gastric groove. In
Figure 1.143 the larger part of the right half, in Figure 1.144 the
larger part of the left half, of the section is omitted. Of the
yelk-sac or remainder of the embryonic vesicle only a small piece of
the wall is indicated below.)

We find in the construction of the ventral wall precisely the same
processes as in the formation of the dorsal wall (Figure 1.137 B,
Figure 1.144 hp, Figure 1.146 bh). It is formed on the flat embryonic
shield of the amniotes from the upper plates of the parietal zone. The
right and left parietal plates bend downwards towards each other, and
grow round the gut in the same way as the gut itself closes. The outer
part of the lateral plates forms the ventral wall or the lower wall of
the body, the two lateral plates bending considerably on the inner
side of the amniotic fold, and growing towards each other from right
and left. While the alimentary canal is closing, the body-wall also
closes on all sides. Hence the ventral wall, which encloses the whole
ventral cavity below, consists of two parts, two lateral plates that
bend towards each other. These approach each other all along, and at
last meet at the navel. We ought, therefore, really to distinguish two
navels, an inner and an outer one. The internal or intestinal navel is
the definitive point of the closing of the gut wall, which puts an end
to the open communication between the ventral cavity and the cavity of
the yelk-sac (Figure 1.105). The external navel in the skin is the
definitive point of the closing of the ventral wall; this is visible
in the developed body as a small depression.

(FIGURE 1.147. Median longitudinal section of the embryo of a chick
(fifth day of incubation), seen from the right side (head to the
right, tail to the left). Dorsal body dark, with convex outline. d
gut, o mouth, a anus, l lungs, h liver, g mesentery, v auricle of the
heart, k ventricle of the heart, b arch of the arteries, t aorta, c
yelk-sac, m vitelline (yelk) duct, u allantois, r pedicle (stalk) of
the allantois, n amnion, w amniotic cavity, s serous membrane. (From
Baer.))

With the formation of the internal navel and the closing of the
alimentary canal is connected the formation of two cavities, which we
call the capital and the pelvic sections of the visceral cavity. As
the embryonic shield lies flat on the wall of the embryonic vesicle at
first, and only gradually separates from it, its fore and hind ends
are independent in the beginning; on the other hand, the middle part
of the ventral surface is connected with the yelk-sac by means of the
vitelline or umbilical duct (Figure 1.147 m). This leads to a notable
curving of the dorsal surface; the head-end bends downwards towards
the breast and the tail-end towards the belly. We see this very
clearly in the excellent old diagrammatic illustration given by Baer
(Figure 1.147), a median longitudinal section of the embryo of the
chick, in which the dorsal body or episoma is deeply shaded. The
embryo seems to be trying to roll up, like a hedgehog protecting
itself from its pursuers. This pronounced curve of the back is due to
the more rapid growth of the convex dorsal surface, and is directly
connected with the severance of the embryo from the yelk-sac. The
further bending of the embryo leads to the formation of the
"head-cavity" of the gut (Figure 1.148 above D) and a similar one at
the tail, known as its "pelvic cavity."

As a result of these processes the embryo attains a shape that may be
compared to a wooden shoe, or, better still, to an overturned canoe.
Imagine a canoe or boat with both ends rounded and a small covering
before and behind; if this canoe is turned upside down, so that the
curved keel is uppermost, we have a fair picture of the canoe-shaped
embryo (Figure 1.147). The upturned convex keel corresponds to the
middle line of the back; the small chamber underneath the fore-deck
represents the capital cavity, and the small chamber under the
rear-deck the pelvic chamber of the gut (cf. Figure 1.140).

The embryo now, as it were, presses into the outer surface of the
embryonic vesicle with its free ends, while it moves away from it with
its middle part. As a result of this change the yelk-sac becomes
henceforth only a pouch-like outer appendage at the middle of the
ventral wall. The ventral appendage, growing smaller and smaller, is
afterwards called the umbilical (navel) vesicle. The cavity of the
yelk-sac or umbilical vesicle communicates with the corresponding
visceral cavity by a wide opening, which gradually contracts into a
narrow and long canal, the vitelline (yelk) duct (ductus vitellinus,
Figure 1.147 m). Hence, if we were to imagine ourselves in the cavity
of the yelk-sac, we could get from it through the yelk-duct into the
middle and still wide open part of the alimentary canal. If we were to
go forward from there into the head-part of the embryo, we should
reach the capital cavity of the gut, the fore-end of which is closed
up.

The reader will ask: "Where are the mouth and the anus?" These are not
at first present in the embryo. The whole of the primitive gut-cavity
is completely closed, and is merely connected in the middle by the
vitelline duct with the equally closed cavity of the embryonic vesicle
(Figure 1.140). The two later apertures of the alimentary canal - the
anus and the mouth - are secondary constructions, formed from the outer
skin. In the horn-plate, at the spot where the mouth is found
subsequently, a pit-like depression is formed, and this grows deeper
and deeper, pushing towards the blind fore-end of the capital cavity;
this is the mouth-pit. In the same way, at the spot in the outer skin
where the anus is afterwards situated a pit-shaped depression appears,
grows deeper and deeper, and approaches the blind hind-end of the
pelvic cavity; this is the anus-pit. In the end these pits touch with
their deepest and innermost points the two blind ends of the primitive
alimentary canal, so that they are now only separated from them by
thin membranous partitions. This membrane finally disappears, and
henceforth the alimentary canal opens in front at the mouth and in the
rear by the anus (Figures 1.141 and 1.147). Hence at first, if we
penetrate into these pits from without, we find a partition cutting
them off from the cavity of the alimentary canal, which gradually
disappears. The formation of mouth and anus is secondary in all the
vertebrates.

(FIGURE 1.148. Longitudinal section of the fore half of a chick-embryo
at the end of the first day of incubation (seen from the left side). k
head-plates, ch chorda. Above it is the blind fore-end of the ventral
tube (m); below it the capital cavity of the gut. d gut-gland layer,
df gut-fibre layer, h horn plate, hh cavity of the heart, hk
heart-capsule, ks head-sheath, kk head-capsule. (From Remak.))

During the important processes which lead to the formation of the
navel, and of the intestinal wall and ventral wall, we find a number
of other interesting changes taking place in the embryonic shield of
the amniotes. These relate chiefly to the prorenal ducts and the first
blood-vessels. The prorenal (primitive kidney) ducts, which at first
lie quite flat under the horn-plate or epiderm (Figure 1.93 ung), soon
back towards each other in consequence of special growth movements
(Figures 1.143 to 1.145 ung). They depart more and more from their
point of origin, and approach the gut-gland layer. In the end they lie
deep in the interior, on either side of the mesentery, underneath the
chorda, (Figure 1.145 ung). At the same time, the two primitive aortas
change their position (cf. Figures 1.138 to 1.145 ao); they travel
inwards underneath the chorda, and there coalesce at last to form a
single secondary aorta, which is found under the rudimentary vertebral
column (Figure 1.145 ao). The cardinal veins, the first venous
blood-vessels, also back towards each other, and eventually unite
immediately above the rudimentary kidneys (Figures 1.145 vc, 152 cav).
In the same spot, at the inner side of the fore-kidneys, we soon see
the first trace of the sexual organs. The most important part of this
apparatus (apart from all its appendages) is the ovary in the female
and the testicle in the male. Both develop from a small part of the
cell-lining of the body-cavity, at the spot where the skin-fibre layer
and gut-fibre layer touch. The connection of this embryonic gland with
the prorenal ducts, which lie close to it and assume most important
relations to it, is only secondary.

(FIGURE 1.149. Longitudinal section of a human embryo of the fourth
week, one-fifth of an inch long, magnified fifteen times. Showing:
bend of skull, yelk-sac, umbilical cord, terminal gut, rudimentary
kidneys, mesoderm, head-gut (with gill-clefts), primitive lungs,
liver, stomach, pancreas, mesentery, primitive kidneys, allantoic
duct, rectum. (From Kollmann.)

FIGURE 1.150. Transverse section of a human embryo of fourteen days.
mr medullary tube, ch chorda. vu umbilical vein, mt myotome, mp middle
plate, ug prorenal duct, lh body-cavity, e ectoderm, bh ventral skin,
hf skin-fibre layer, df gut-fibre layer. (From Kollmann.)

FIGURE 1.151. Transverse section of a shark-embryo (or young
selachius). mr medullary tube, ch chorda, a aorta, d gut, vp principal
(or subintestinal) vein, mt myotome, mm muscular mass of the
provertebra, mp middle plate, ug prorenal duct, lh body-cavity, e
ectoderm of the rudimentary extremities, mz mesenchymic cells, z point
where the myotome and nephrotome separate. (From H.E. Ziegler.)

FIGURE 1.152. Transverse section of a duck-embryo with twenty-four
primitive segments. (From Balfour.) From a dorsal lateral joint of the
medullary tube (spc) the spinal ganglia (spg) grow out between it and
the horn-plate. ch chorda, ao double aorta, hy gut-gland layer, sp
gut-fibre layer, with blood-vessels in section, ms muscle plate, in
the dorsal wall of the myocoel (episomite). Below the cardinal vein
(cav) is the prorenal duct (wd) and a segmental prorenal canal (st).
The skin-fibre layer of the body-wall (so) is continued in the
amniotic fold (am). Between the four secondary germinal layers and the
structures formed from them there is formed embryonic connective
matter with stellate cells and vascular structures (Hertwig's
"mesenchym").)


CHAPTER 1.14. THE ARTICULATION OF THE BODY.*

(* The term articulation is used in this chapter to denote both
"segmentation" and "articulation" in the ordinary sense. - Translator.)

The vertebrate stem, to which our race belongs as one of the latest
and most advanced outcomes of the natural development of life, is
rightly placed at the head of the animal kingdom. This privilege must
be accorded to it, not only because man does in point of fact soar far
above all other animals, and has been lifted to the position of "lord
of creation"; but also because the vertebrate organism far surpasses
all the other animal-stems in size, in complexity of structure, and in
the advanced character of its functions. From the point of view of
both anatomy and physiology, the vertebrate stem outstrips all the
other, or invertebrate, animals.

There is only one among the twelve stems of the animal kingdom that
can in many respects be compared with the vertebrates, and reaches an
equal, if not a greater, importance in many points. This is the stem
of the articulates, composed of three classes: 1, the annelids
(earth-worms, leeches, and cognate forms); 2, the crustacea (crabs,
etc.); 3, the tracheata (spiders, insects, etc.). The stem of the
articulates is superior not only to the vertebrates, but to all other
animal-stems, in variety of forms, number of species, elaborateness of
individuals, and general importance in the economy of nature.

When we have thus declared the vertebrates and the articulates to be
the most important and most advanced of the twelve stems of the animal
kingdom, the question arises whether this special position is accorded
to them on the ground of a peculiarity of organisation that is common
to the two. The answer is that this is really the case; it is their
segmental or transverse articulation, which we may briefly call
metamerism. In all the vertebrates and articulates the developed
individual consists of a series of successive members (segments or
metamera = "parts"); in the embryo these are called primitive segments
or somites. In each of these segments we have a certain group of
organs reproduced in the same arrangement, so that we may regard each
segment as an individual unity, or a special "individual" subordinated
to the entire personality.

The similarity of their segmentation, and the consequent physiological
advance in the two stems of the vertebrates and articulates, has led
to the assumption of a direct affinity between them, and an attempt to
derive the former directly from the latter. The annelids were supposed
to be the direct ancestors, not only of the crustacea and tracheata,
but also of the vertebrates. We shall see later (Chapter 2.20) that
this annelid theory of the vertebrates is entirely wrong, and ignores
the most important differences in the organisation of the two stems.
The internal articulation of the vertebrates is just as profoundly
different from the external metamerism of the articulates as are their
skeletal structure, nervous system, vascular system, and so on. The
articulation has been developed in a totally different way in the two
stems. The unarticulated chordula (Figures 1.83 to 1.86), which we
have recognised as one of the chief palingenetic embryonic forms of
the vertebrate group, and from which we have inferred the existence of
a corresponding ancestral form for all the vertebrates and tunicates,
is quite unthinkable as the stem-form of the articulates.

All articulated animals came originally from unarticulated ones. This
phylogenetic principle is as firmly established as the ontogenetic
fact that every articulated animal-form develops from an unarticulated
embryo. But the organisation of the embryo is totally different in the
two stems. The chordula-embryo of all the vertebrates is characterised
by the dorsal medullary tube, the neurenteric canal, which passes at
the primitive mouth into the alimentary canal, and the axial chorda
between the two. None of the articulates, either annelids or
arthropods (crustacea and tracheata), show any trace of this type of
organisation. Moreover, the development of the chief systems of organs
proceeds in the opposite way in the two stems. Hence the segmentation
must have arisen independently in each. This is not at all surprising;
we find analogous cases in the stalk-articulation of the higher plants
and in several groups of other animal stems.

The characteristic internal articulation of the vertebrates and its
importance in the organisation of the stem are best seen in the study
of the skeleton. Its chief and central part, the cartilaginous or bony
vertebral column, affords an obvious instance of vertebrate
metamerism; it consists of a series of cartilaginous or bony pieces,
which have long been known as vertebrae (or spondyli). Each vertebra
is directly connected with a special section of the muscular system,
the nervous system, the vascular system, etc. Thus most of the "animal
organs" take part in this vertebration. But we saw, when we were
considering our own vertebrate character (in Chapter 1.11), that the
same internal articulation is also found in the lowest primitive
vertebrates, the acrania, although here the whole skeleton consists
merely of the simple chorda, and is not at all articulated. Hence the
articulation does not proceed primarily from the skeleton, but from
the muscular system, and is clearly determined by the more advanced
swimming-movements of the primitive chordonia-ancestors.

(FIGURES 1.153 TO 1.155. Sole-shaped embryonic disk of the chick, in
three successive stages of development, looked at from the dorsal
surface, magnified about twenty times, somewhat diagrammatic. Figure
1.153 with six pairs of somites. Brain a simple vesicle (hb).
Medullary furrow still wide open from x; greatly widened at z. mp
medullary plates, sp lateral plates, y limit of gullet-cavity (sh) and
fore-gut (vd). Figure 1.154 with ten pairs of somites. Brain divided
into three vesicles: v fore-brain, m middle-brain, h hind-brain, c
heart, dv vitelline-veins. Medullary furrow still wide open behind
(z). mp medullary plates. Figure 1.155 with sixteen pairs of somites.
Brain divided into five vesicles: v fore-brain, z intermediate-brain,
m middle-brain, h hind-brain, n after-brain, a optic vesicles, g
auditory vesicles, c heart, dv vitelline veins, mp medullary plate, uw
primitive vertebra.)

It is, therefore, wrong to describe the first rudimentary segments in
the vertebrate embryo as primitive vertebrae or provertebrae; the fact
that they have been so called for some time has led to much error and
misunderstanding. Hence we shall give the name of "somites" or
primitive segments to these so-called "primitive vertebrae." If the
latter name is retained at all, it should only be used of the
sclerotom - i.e., the small part of the somites from which the later
vertebra does actually develop.

Articulation begins in all vertebrates at a very early embryonic
stage, and this indicates the considerable phylogenetic age of the
process. When the chordula (Figures 1.83 to 1.86) has completed its
characteristic composition, often even a little earlier, we find in
the amniotes, in the middle of the sole-shaped embryonic shield,
several pairs of dark square spots, symmetrically distributed on both
sides of the chorda (Figures 1.131 to 1.135). Transverse sections
(Figure 1.93 uw) show that they belong to the stem-zone (episoma) of
the mesoderm, and are separated from the parietal zone (hyposoma) by
the lateral folds; in section they are still quadrangular, almost
square, so that they look something like dice. These pairs of "cubes"


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