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Ernst Heinrich Philipp August Haeckel.

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coelomula of the human embryo (Figure 1.97), by transverse sections
through its primitive mouth, that its two coelom-pouches are developed
in just the same way as in the rabbit (Figure 1.96); moreover, the
peculiar course of the gastrulation is just the same.

(FIGURE 1.120. Embryonic vesicle of a seven-days-old rabbit with oval
embryonic shield (ag).
A seen from above, B from the side. (From Kolliker.) ag dorsal shield
or embryonic spot. In B the upper half of the vesicle is made up of
the two primary germinal layers, the lower (up to ge) only from the
outer layer.)

The germinative area forms in the human embryo in the same way as in
the other mammals, and in the middle part of this we have the
embryonic shield, the purport of which we considered in Chapter 1.12.
The next changes in the embryonic disk, or the "embryonic spot," take
place in corresponding fashion. These are the changes we are now going
to consider more closely.

The chief part of the oval embryonic shield is at first the narrow
hinder end; it is in the middle line of this that the primitive streak
appears (Figure 1.121 ps). The narrow longitudinal groove in it - the
so-called "primitive groove" - is, as we have seen, the primitive mouth
of the gastrula. In the gastrula-embryos of the mammals, which are
much modified cenogenetically, this cleft-shaped prostoma is
lengthened so much that it soon traverses the whole of the hinder half
of the dorsal shield; as we find in a rabbit embryo of six to eight
days (Figure 1.122 pr). The two swollen parallel borders that limit
this median furrow are the side lips of the primitive mouth, right and
left. In this way the bilateral-symmetrical type of the vertebrate
becomes pronounced. The subsequent head of the amniote is developed
from the broader and rounder fore-half of the dorsal shield.

In this fore-half of the dorsal shield a median furrow quickly makes
its appearance (Figure 1.123 rf). This is the broader dorsal furrow or
medullary groove, the first beginning of the central nervous system.
The two parallel dorsal or medullary swellings that enclose it grow
together over it afterwards, and form the medullary tube. As is seen
in transverse sections, it is formed only of the outer germinal layer
(Figures 1.95 and 1.136). The lips of the primitive mouth, however,
lie, as we know, at the important point where the outer layer bends
over the inner, and from which the two coelom pouches grow between the
primary germinal layers.

(FIGURE 1.121. Oval embryonic shield of the rabbit (A of six days
eighteen hours, B of eight days). (From Kolliker.) ps primitive
streak, pr primitive groove, arg area germinalis, sw sickle-shaped
germinal growth.

FIGURE 1.122. Dorsal shield (ag) and germinative area of a
rabbit-embryo of eight days. (From Kolliker.) pr primitive groove, rf
dorsal furrow.

FIGURE 1.123. Embryonic shield of a rabbit of eight days. (From Van
Beneden.) pr primitive groove, cn canalis neurentericus, nk nodus
neurentericus (or "Hensen's ganglion"), kf head-process (chorda).

FIGURE 1.124. Longitudinal section of the coelomula of amphioxus (from
the left). i entoderm, d primitive gut, cn medullary duct, n nerve
tube, m mesoderm, s first primitive segment, c coelom-pouches. (From
Hatschek.))

Thus the median primitive furrow (pr) in the hind-half and the median
medullary furrow (rf) in the fore-half of the oval shield are totally
different structures, although the latter seems to a superficial
observer to be merely the forward continuation of the former. Hence
they were formerly always confused. This error was the more pardonable
as immediately afterwards the two grooves do actually pass into each
other in a very remarkable way. The point of transition is the
remarkable neurenteric canal (Figure 1.124 cn). But the direct
connection which is thus established does not last long; the two are
soon definitely separated by a partition.

The enigmatic neurenteric canal is a very old embryonic organ, and of
great phylogenetic interest, because it arises in the same way in all
the chordonia (both tunicates and vertebrates). In every case it
touches or embraces like an arch the posterior end of the chorda,
which has been developed here in front out of the middle line of the
primitive gut (between the two coelom-folds of the sickle groove)
("head-process," Figure 1.123 kf). These very ancient and strictly
hereditary structures, which have no physiological significance
to-day, deserve (as "rudimentary organs") our closest attention. The
tenacity with which the useless neurenteric canal has been transmitted
down to man through the whole series of vertebrates is of equal
interest for the theory of descent in general, and the phylogeny of
the chordonia in particular.

The connection which the neurenteric canal (Figure 1.123 cn)
establishes between the dorsal nerve-tube (n) and the ventral gut-tube
(d) is seen very plainly in the amphioxus in a longitudinal section of
the coelomula, as soon as the primitive mouth is completely closed at
its hinder end. The medullary tube has still at this stage an opening
at the forward end, the neuroporus (Figure 1.83 np). This opening also
is afterwards closed. There are then two completely closed canals over
each other - the medullary tube above and the gastric tube below, the
two being separated by the chorda. The same features as in the acrania
are exhibited by the related tunicates, the ascidiae.

Again, we find the neurenteric canal in just the same form and
situation in the amphibia. A longitudinal section of a young tadpole
(Figure 1.125) shows how we may penetrate from the still open
primitive mouth (x) either into the wide primitive gut-cavity (al) or
the narrow overlying nerve-tube. A little later, when the primitive
mouth is closed, the narrow neurenteric canal (Figure 1.126 ne)
represents the arched connection between the dorsal medullary canal
(mc) and the ventral gastric canal.

(FIGURE 1.125. Longitudinal section of the chordula of a frog. (From
Balfour.) nc nerve-tube, x canalis neurentericus, al alimentary canal,
yk yelk-cells, m mesoderm.

FIGURE 1.126. Longitudinal section of a frog-embryo. (From Goette.) m
mouth, l liver, an anus, ne canalis neurentericus, mc medullary-tube,
pn pineal body (epiphysis), ch chorda.

FIGURES 1.127 AND 1.128. Dorsal shield of the chick. (From Balfour.)
The medullary furrow (me), which is not yet visible in Figure 1.130,
encloses with its hinder end the fore end of the primitive groove (pr)
in Figure 1.131.)

In the amniotes this original curved form of the neurenteric canal
cannot be found at first, because here the primitive mouth travels
completely over to the dorsal surface of the gastrula, and is
converted into the longitudinal furrow we call the primitive groove.
Hence the primitive groove (Figure 1.128 pr), examined from above,
appears to be the straight continuation of the fore-lying and younger
medullary furrow (me). The divergent hind legs of the latter embrace
the anterior end of the former. Afterwards we have the complete
closing of the primitive mouth, the dorsal swellings joining to form
the medullary tube and growing over it. The neurenteric canal then
leads directly, in the shape of a narrow arch-shaped tube (Figure
1.129 ne), from the medullary tube (sp) to the gastric tube (pag).
Directly in front of it is the latter end of the chorda (cli).

While these important processes are taking place in the axial part of
the dorsal shield, its external form also is changing. The oval form
(Figure 1.117) becomes like the sole of a shoe or sandal, lyre-shaped
or finger-biscuit shaped (Figure 1.130). The middle third does not
grow in width as quickly as the posterior, and still less than the
anterior third; thus the shape of the permanent body becomes somewhat
narrow at the waist. At the same time, the oval form of the
germinative area returns to a circular shape, and the inner pellucid
area separates more clearly from the opaque outer area (Figure 1.131
a). The completion of the circle in the area marks the limit of the
formation of blood-vessels in the mesoderm.

(FIGURE 1.129. Longitudinal section of the hinder end of a chick.
(From Balfour.) sp medullary tube, connected with the terminal gut
(pag) by the neurenteric canal (ne), ch chorda, pr neurenteric (or
Hensen's) ganglion, al allantois, ep ectoderm, hy entoderm, so
parietal layer, sp visceral layer, an anus-pit, am amnion.)

The characteristic sandal-shape of the dorsal shield, which is
determined by the narrowness of the middle part, and which is compared
to a violin, lyre, or shoe-sole, persists for a long time in all the
amniotes. All mammals, birds, and reptiles have substantially the same
construction at this stage, and even for a longer or shorter period
after the division of the primitive segments into the coelom-folds has
begun (Figure 1.132). The human embryonic shield assumes the
sandal-form in the second week of development; towards the end of the
week our sole-shaped embryo has a length of about one-twelfth of an
inch (Figure 1.133).

The complete bilateral symmetry of the vertebrate body is very early
indicated in the oval form of the embryonic shield (Figure 1.117) by
the median primitive streak; in the sandal-form it is even more
pronounced (Figures 1.131 to 1.135). In the lateral parts of the
embryonic shield a darker central and a lighter peripheral zone become
more obvious; the former is called the stem-zone (Figure 1.134 stz),
and the latter the parietal zone (pz); from the first we get the
dorsal and from the second the ventral half of the body-wall. The
stem-zone of the amniote embryo would be called more appropriately the
dorsal zone or dorsal shield; from it develops the whole of the dorsal
half of the later body (or permanent body) - that is to say, the dorsal
body (episoma). Again, it would be better to call the "parietal zone"
the ventral zone or ventral shield; from it develop the ventral
"lateral plates," which afterwards separate from the embryonic vesicle
and form the ventral body (hyposoma) - that is to say, the ventral half
of the permanent body, together with the body-cavity and the gastric
canal that it encloses.

(FIGURE 1.130. Germinal area or germinal disk of the rabbit, with
sole-shaped embryonic shield, magnified about ten times. The clear
circular field (d) is the opaque area. The pellucid area (c) is
lyre-shaped, like the embryonic shield itself (b). In its axis is seen
the dorsal furrow or medullary furrow (a). (From Bischoff.))

The sole-shaped germinal shields of all the amniotes are still, at the
stage of construction which Figure 1.134 illustrates in the rabbit and
Figure 1.135 in the opossum, so like each other that we can either not
distinguish them at all or only by means of quite subordinate
peculiarities in the size of the various parts. Moreover, the human
sandal-shaped embryo cannot at this stage be distinguished from those
of other mammals, and it particularly resembles that of the rabbit. On
the other hand, the outer form of these flat sandal-shaped embryos is
very different from the corresponding form of the lower animals,
especially the acrania (amphioxus). Nevertheless, the body is just the
same in the essential features of its structure as that we find in the
chordula of the latter (Figures 1.83 to 1.86), and in the embryonic
forms which immediately develop from it. The striking external
difference is here again due to the fact that in the palingenetic
embryos of the amphioxus (Figures 1.83 and 1.84) and the amphibia
(Figures 1.85 and 1.86) the gut-wall and body-wall form closed tubes
from the first, whereas in the cenogenetic embryos of the amniotes
they are forced to expand leaf-wise on the surface owing to the great
extension of the food-yelk.

(FIGURE 1.131. Embryo of the opossum, sixty hours old, one-sixth of an
inch in diameter. (From Selenka) b the globular embryonic vesicle, a
the round germinative area, b limit of the ventral plates, r dorsal
shield, v its fore part, u the first primitive segment, ch chorda, chr
its fore-end, pr primitive groove (or mouth).

FIGURE 1.132. Sandal-shaped embryonic shield of a rabbit of eight
days, with the fore part of the germinative area (ao opaque, ap
pellucid area). (From Kolliker.) rf dorsal furrow, in the middle of
the medullary plate, h, pr primitive groove (mouth), stz dorsal (stem)
zone, pz ventral (parietal) zone. In the narrow middle part the first
three primitive segments may be seen.)

It is all the more notable that the early separation of dorsal and
ventral halves takes place in the same rigidly hereditary fashion in
all the vertebrates. In both the acrania and the craniota the dorsal
body is about this period separated from the ventral body. In the
middle part of the body this division has already taken place by the
construction of the chorda between the dorsal nerve-tube and the
ventral canal. But in the outer or lateral part of the body it is only
brought about by the division of the coelom-pouches into two
sections - a dorsal episomite (dorsal segment or provertebra) and a
ventral hyposomite (or ventral segment) by a frontal constriction. In
the amphioxus each of the former makes a muscular pouch, and each of
the latter a sex-pouch or gonad.

These important processes of differentiation in the mesoderm, which we
will consider more closely in the next chapter, proceed step by step
with interesting changes in the ectoderm, while the entoderm changes
little at first. We can study these processes best in transverse
sections, made vertically to the surface through the sole-shaped
embryonic shield. Such a transverse section of a chick embryo, at the
end of the first day of incubation, shows the gut-gland layer as a
very simple epithelium, which is spread like a leaf over the outer
surface of the food-yelk (Figure 1.92). The chorda (ch) has separated
from the dorsal middle line of the entoderm; to the right and left of
it are the two halves of the mesoderm, or the two coelom-folds. A
narrow cleft in the latter indicates the body-cavity (uwh); this
separates the two plates of the coelom-pouches, the lower (visceral)
and upper (parietal). The broad dorsal furrow (Rf) formed by the
medullary plate (m) is still wide open, but is divided from the
lateral horn-plate (h) by the parallel medullary swellings, which
eventually close.

(FIGURE 1.133. Human embryo at the sandal-stage, one-twelfth of an
inch long, from the end of the second week, magnified twenty-five
times. (From Count Spee.)

FIGURE 1.134. Sandal-shaped embryonic shield of a rabbit of nine days.
(From Kolliker.) (Back view from above.) stz stem-zone or dorsal
shield (with eight pairs of primitive segments), pz parietal or
ventral zone, ap pellucid area, af amnion-fold, h heart, ph
pericardial cavity, vo omphalo-mesenteric vein, ab eye-vesicles, vh
fore brain, mh middle brain, hh hind brain, uw primitive segments (or
vertebrae).)

During these processes important changes are taking place in the outer
germinal layer (the "skin-sense layer"). The continued rise and growth
of the dorsal swellings causes their higher parts to bend together at
their free borders, approach nearer and nearer (Figure 1.136 w), and
finally unite. Thus in the end we get from the open dorsal furrow, the
upper cleft of which becomes narrower and narrower, a closed
cylindrical tube (Figure 1.137 mr). This tube is of the utmost
importance; it is the beginning of the central nervous system, the
brain and spinal marrow, the medullary tube. This embryonic fact was
formerly looked upon as very mysterious. We shall see presently that
in the light of the theory of descent it is a thoroughly natural
process. The phylogenetic explanation of it is that the central
nervous system is the organ by means of which all intercourse with the
outer world, all psychic action and sense-perception, are
accomplished; hence it was bound to develop originally from the outer
and upper surface of the body, or from the outer skin. The medullary
tube afterwards separates completely from the outer germinal layer,
and is surrounded by the middle parts of the provertebrae and forced
inwards (Figure 1.146). The remaining portion of the skin-sense layer
(Figure 1.93 h) is now called the horn-plate or horn-layer, because
from it is developed the whole of the outer skin or epidermis, with
all its horny appendages (nails, hair, etc.).

(FIGURE 1.135. Sandal-shaped embryonic shield of an opossum
(Didelphys), three days old. (From Selenka.) (Back view from above.)
stz stem-zone or dorsal shield (with eight pairs of primitive
segments), pz parietal or ventral zone, ap pellucid area, ao opaque
area, hh halves of the heart, v fore-end, h hind-end. In the median
line we see the chorda (ch) through the transparent medullary tube
(m). u primitive segment, pr primitive streak (or primitive mouth).)

A totally different organ, the prorenal (primitive kidney) duct (ung),
is found to be developed at an early stage from the ectoderm. This is
originally a quite simple, tube-shaped, lengthy duct, or straight
canal, which runs from front to rear at each side of the provertebrae
(on the outer side, Figure 1.93 ung). It originates, it seems, out of
the horn-plate at the side of the medullary tube, in the gap that we
find between the provertebral and the lateral plates. The prorenal
duct is visible in this gap even at the time of the severance of the
medullary tube from the horn-plate. Other observers think that the
first trace of it does not come from the skin-sense layer, but the
skin-fibre layer.

The inner germinal layer, or the gut-fibre layer (Figure 1.93 dd),
remains unchanged during these processes. A little later, however, it
shows a quite flat, groove-like depression in the middle line of the
embryonic shield, directly under the chorda. This depression is called
the gastric groove or furrow. This at once indicates the future lot of
this germinal layer. As this ventral groove gradually deepens, and its
lower edges bend towards each other, it is formed into a closed tube,
the alimentary canal, in the same way as the medullary groove grows
into the medullary tube. The gut-fibre layer (Figure 1.137 f), which
lies on the gut-gland layer (d), naturally follows it in its folding.
Moreover, the incipient gut-wall consists from the first of two
layers, internally the gut-gland layer and externally the gut-fibre
layer.

The formation of the alimentary canal resembles that of the medullary
tube to this extent - in both cases a straight groove or furrow arises
first of all in the middle line of a flat layer. The edges of this
furrow then bend towards each other, and join to form a tube (Figure
1.137). But the two processes are really very different. The medullary
tube closes in its whole length, and forms a cylindrical tube, whereas
the alimentary canal remains open in the middle, and its cavity
continues for a long time in connection with the cavity of the
embryonic vesicle. The open connection between the two cavities is
only closed at a very late stage, by the construction of the navel.
The closing of the medullary tube is effected from both sides, the
edges of the groove joining together from right and left. But the
closing of the alimentary canal is not only effected from right and
left, but also from front and rear, the edges of the ventral groove
growing together from every side towards the navel. Throughout the
three higher classes of vertebrates the whole of this process of the
construction of the gut is closely connected with the formation of the
navel, or with the separation of the embryo from the yelk-sac or
umbilical vesicle.

In order to get a clear idea of this, we must understand carefully the
relation of the embryonic shield to the germinative area and the
embryonic vesicle. This is done best by a comparison of the five
stages which are shown in longitudinal section in Figures 1.138 to
1.142. The embryonic shield (c), which at first projects very slightly
over the surface of the germinative area, soon begins to rise higher
above it, and to separate from the embryonic vesicle. At this point
the embryonic shield, looked at from the dorsal surface, shows still
the original simple sandal-shape (Figures 1.133 to 1.135). We do not
yet see any trace of articulation into head, neck, trunk, etc., or
limbs. But the embryonic shield has increased greatly in thickness,
especially in the anterior part. It now has the appearance of a thick,
oval swelling, strongly curved over the surface of the germinative
area. It begins to sever completely from the embryonic vesicle, with
which it is connected at the ventral surface. As this severance
proceeds, the back bends more and more; in proportion as the embryo
grows the embryonic vesicle decreases, and at last it merely hangs as
a small vesicle from the belly of the embryo (Figure 1.142 ds). In
consequence of the growth-movements which cause this severance, a
groove-shaped depression is formed at the surface of the vesicle, the
limiting furrow, which surrounds the vesicle in the shape of a pit,
and a circular mound or dam (Figure 1.139 ks) is formed at the outside
of this pit by the elevation of the contiguous parts of the germinal
vesicle.

(FIGURE 1.136. Transverse section of the embryonic disk of a chick at
the end of the first day of incubation, magnified about twenty times.
The edges of the medullary plate (m), the medullary swellings (w),
which separate the medullary from the horn-plate (h), are bending
towards each other. At each side of the chorda (ch) the primitive
segment plates (u) have separated from the lateral plates (sp). A
gut-gland layer. (From Remak.))

In order to understand clearly this important process, we may compare
the embryo to a fortress with its surrounding rampart and trench. The
ditch consists of the outer part of the germinative area, and comes to
an end at the point where the area passes into the vesicle. The
important fold of the middle germinal layer that brings about the
formation of the body-cavity spreads beyond the borders of the embryo
over the whole germinative area. At first this middle layer reaches as
far as the germinative area; the whole of the rest of the embryonic
vesicle consists in the beginning only of the two original limiting
layers, the outer and inner germinal layers. Hence, as far as the
germinative area extends the germinal layer splits into the two plates
we have already recognised in it, the outer skin-fibre layer and the
inner gut-fibre layer. These two plates diverge considerably, a clear
fluid gathering between them (Figure 1.140 am). The inner plate, the
gut-fibre layer, remains on the inner layer of the embryonic vesicle
(on the gut-gland layer). The outer plate, the skin-fibre layer, lies
close on the outer layer of the germinative area, or the skin-sense
layer, and separates together with this from the embryonic vesicle.
From these two united outer plates is formed a continuous membrane.
This is the circular mound that rises higher and higher round the
whole embryo, and at last joins above it (Figures 1.139 to 1.142 am).
To return to our illustration of the fortress, we must imagine the
circular rampart to be extraordinarily high and towering far above the
fortress. Its edges bend over like the combs of an overhanging wall of
rock that would enclose the fortress; they form a deep hollow, and at
last join together above. In the end the fortress lies entirely within
the hollow that has been formed by the growth of the edges of this
large rampart.

(FIGURE 1.137. Three diagrammatic transverse sections of the embryonic
disk of the higher vertebrate, to show the origin of the tubular
organs from the bending germinal layers. In Figure A the medullary
tube (n) and the alimentary canal (a) are still open grooves. In
Figure B the medullary tube (n) and the dorsal wall are closed, but
the alimentary canal (a) and the ventral wall are open; the prorenal
ducts (u) are cut off from the horn-plate (h) and internally connected
with segmental prorenal canals. In Figure C both the medullary tube
and the dorsal wall above and the alimentary canal and ventral wall
below are closed. All the open grooves have become closed tubes; the
primitive kidneys are directed inwards. The letters have the same
meaning in all three figures: h skin-sense layer, n medullary tube, u
prorenal ducts, x axial rod, s primitive-vertebra, r dorsal wall, b
ventral wall, c body-cavity or coeloma, f gut-fibre layer, t primitive
artery (aorta), v primitive vein (subintestinal vein), d gut-fibre
layer, a alimentary canal.)

As the two outer layers of the germinative area thus rise in a fold


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