Ernst Heinrich Philipp August Haeckel.

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of the mesoderm are the first traces of the primitive segments or
somites, the so-called "protovertebrae." (Figures 1.153 to 1.155 uw).

(FIGURE 1.156. Embryo of the amphioxus, sixteen hours old, seen from
the back. (From Hatschek.) d primitive gut, u primitive mouth, p polar
cells of the mesoderm, c coelom-pouches, m their first segment, n
medullary tube, i entoderm, e ectoderm, s first segment-fold.

FIGURE 1.157. Embryo of the amphioxus, twenty hours old, with five
somites. (Right view; for left view see Figure 1.124.) (From
Hatschek.) V fore end, H hind end. ak, mk, ik outer, middle, and inner
germinal layers; dh alimentary canal, n neural tube, cn canalis
neurentericus, ush coelom-pouches (or primitive-segment cavities), us1
first (and foremost) primitive segment.)

Among the mammals the embryos of the marsupials have three pairs of
somites (Figure 1.131) after sixty hours, and eight pairs after
seventy-two hours (Figure 1.135). They develop more slowly in the
embryo of the rabbit; this has three somites on the eighth day (Figure
1.132), and eight somites a day later (Figure 1.134). In the incubated
hen's egg the first somites make their appearance thirty hours after
incubation begins (Figure 1.153). At the end of the second day the
number has risen to sixteen or eighteen (Figure 1.155). The
articulation of the stem-zone, to which the somites owe their origin,
thus proceeds briskly from front to rear, new transverse constrictions
of the "protovertebral plates" forming continuously and successively.
The first segment, which is almost half-way down in the embryonic
shield of the amniote, is the foremost of all; from this first somite
is formed the first cervical vertebra with its muscles and skeletal
parts. It follows from this, firstly, that the multiplication of the
primitive segments proceeds backwards from the front, with a constant
lengthening of the hinder end of the body; and, secondly, that at the
beginning of segmentation nearly the whole of the anterior half of the
sole-shaped embryonic shield of the amniote belongs to the later head,
while the whole of the rest of the body is formed from its hinder
half. We are reminded that in the amphioxus (and in our hypothetic
primitive vertebrate, Figures 1.98 to 1.102) nearly the whole of the
fore half corresponds to the head, and the hind half to the trunk.

The number of the metamera, and of the embryonic somites or primitive
segments from which they develop, varies considerably in the
vertebrates, according as the hind part of the body is short or is
lengthened by a tail. In the developed man the trunk (including the
rudimentary tail) consists of thirty-three metamera, the solid centre
of which is formed by that number of vertebrae in the vertebral column
(seven cervical, twelve dorsal, five lumbar, five sacral, and four
caudal). To these we must add at least nine head-vertebrae, which
originally (in all the craniota) constitute the skull. Thus the total
number of the primitive segments of the human body is raised to at
least forty-two; it would reach forty-five to forty-eight if
(according to recent investigations) the number of the original
segments of the skull is put at twelve to fifteen. In the tailless or
anthropoid apes the number of metamera is much the same as in man,
only differing by one or two; but it is much larger in the long-tailed
apes and most of the other mammals. In long serpents and fishes it
reaches several hundred (sometimes 400).

(FIGURES 1.158 TO 1.160. Embryo of the amphioxus, twenty four hours
old, with eight somites. (From Hatschek.) Figures 1.158 and 1.159
lateral view (from left). Figure 1.160 seen from back. In Figure 1.158
only the outlines of the eight primitive segments are indicated, in
Figure 1.159 their cavities and muscular walls. V fore end, H hind
end, d gut, du under and dd upper wall of the gut, ne canalis
neurentericus, nv ventral, nd dorsal wall of the neural tube, np
neuroporus, dv fore pouch of the gut, ch chorda, mf mesodermic fold,
pm polar cells of the mesoderm (ms), e ectoderm.)

In order to understand properly the real nature and origin of
articulation in the human body and that of the higher vertebrates, it
is necessary to compare it with that of the lower vertebrates, and
bear in mind always the genetic connection of all the members of the
stem. In this the simple development of the invaluable amphioxus once
more furnishes the key to the complex and cenogenetically modified
embryonic processes of the craniota. The articulation of the amphioxus
begins at an early stage - earlier than in the craniotes. The two
coelom-pouches have hardly grown out of the primitive gut (Figure
1.156 c) when the blind fore part of it (farthest away from the
primitive mouth, u) begins to separate by a transverse fold (s): this
is the first primitive segment. Immediately afterwards the hind part
of the coelom-pouches begins to divide into a series of pieces by new
transverse folds (Figure 1.157). The foremost of these primitive
segments (us1) is the first and oldest; in Figures 1.124 and 1.157
there are already five formed. They separate so rapidly, one behind
the other, that eight pairs are formed within twenty-four hours of the
beginning of development, and seventeen pairs twenty-four hours later.
The number increases as the embryo grows and extends backwards, and
new cells are formed constantly (at the primitive mouth) from the two
primitive mesodermic cells (Figures 1.159 to 1.160).

(FIGURES 1.161 AND 1.162. Transverse section of shark-embryos (through
the region of the kidneys). (From Wijhe and Hertwig.) In Figure 1.162
the dorsal segment-cavities (h) are already separated from the
body-cavity (lh), but they are connected a little earlier (Figure
1.161), nr neural tube, ch chorda, sch subchordal string, ao aorta, sk
skeletal-plate, mp muscle-plate, cp cutis-plate, w connection of
latter (growth-zone), vn primitive kidneys, ug prorenal duct, uk
prorenal canals, us point where they are cut off, tr prorenal funnel,
mk middle germ-layer (mk1 parietal, mk2 visceral), ik inner germ-layer
(gut-gland layer).)

This typical articulation of the two coelom-sacs begins very early in
the lancelet, before they are yet severed from the primitive gut, so
that at first each segment-cavity (us) still communicates by a narrow
opening with the gut, like an intestinal gland. But this opening soon
closes by complete severance, proceeding regularly backwards. The
closed segments then extend more, so that their upper half grows
upwards like a fold between the ectoderm (ak) and neural tube (n), and
the lower half between the ectoderm and alimentary canal (ch; Figure
1.82 d, left half of the figure). Afterwards the two halves completely
separate, a lateral longitudinal fold cutting between them (mk, right
half of Figure 1.82). The dorsal segments (sd) provide the muscles of
the trunk the whole length of the body (1.159): this cavity afterwards
disappears. On the other hand, the ventral parts give rise, from their
uppermost section, to the pronephridia or primitive-kidney canals, and
from the lower to the segmental rudiments of the sexual glands or
gonads. The partitions of the muscular dorsal pieces (myotomes)
remain, and determine the permanent articulation of the vertebrate
organism. But the partitions of the large ventral pieces (gonotomes)
become thinner, and afterwards disappear in part, so that their
cavities run together to form the metacoel, or the simple permanent

The articulation proceeds in substantially the same way in the other
vertebrates, the craniota, starting from the coelom-pouches. But
whereas in the former case there is first a transverse division of the
coelom-sacs (by vertical folds) and then the dorso-ventral division,
the procedure is reversed in the craniota; in their case each of the
long coelom-pouches first divides into a dorsal (primitive segment
plates) and a ventral (lateral plates) section by a lateral
longitudinal fold. Only the former are then broken up into primitive
segments by the subsequent vertical folds; while the latter (segmented
for a time in the amphioxus) remain undivided, and, by the divergence
of their parietal and visceral plates, form a body-cavity that is
unified from the first. In this case, again, it is clear that we must
regard the features of the younger craniota as cenogenetically
modified processes that can be traced palingenetically to the older

We have an interesting intermediate stage between the acrania and the
fishes in these and many other respects in the cyclostoma (the hag and
the lamprey, cf. Chapter 2.21).

(FIGURE 1.163. Frontal (or horizontal-longitudinal) section of a
triton-embryo with three pairs of primitive segments. ch chorda, us
primitive segments, ush their cavity, ak horn plate.)

Among the fishes the selachii, or primitive fishes, yield the most
important information on these and many other phylogenetic questions
(Figures 1.161 and 1.162). The careful studies of Ruckert, Van Wijhe,
H.E. Ziegler, and others, have given us most valuable results. The
products of the middle germinal layer are partly clear in these cases
at the period when the dorsal primitive segment cavities (or myocoels,
h) are still connected with the ventral body-cavity (lh; Figure
1.161). In Figure 1.162, a somewhat older embryo, these cavities are
separated. The outer or lateral wall of the dorsal segment yields the
cutis-plate (cp), the foundation of the connective corium. From its
inner or median wall are developed the muscle-plate (mp, the rudiment
of the trunk-muscles) and the skeletal plate, the formative matter of
the vertebral column (sk).

In the amphibia, also, especially the water-salamander (Triton), we
can observe very clearly the articulation of the coelom-pouches and
the rise of the primitive segments from their dorsal half (cf. Figure
1.91, A, B, C). A horizontal longitudinal section of the
salamander-embryo (Figure 1.163) shows very clearly the series of
pairs of these vesicular dorsal segments, which have been cut off on
each side from the ventral side-plates, and lie to the right and left
of the chorda.

(FIGURE 1.164. The third cervical vertebra (human).

FIGURE 1.165. The sixth dorsal vertebra (human).

FIGURE 1.166. The second lumbar vertebra (human).)

The metamerism of the amniotes agrees in all essential points with
that of the three lower classes of vertebrates we have considered; but
it varies considerably in detail, in consequence of cenogenetic
disturbances that are due in the first place (like the degeneration of
the coelom-pouches) to the large development of the food-yelk. As the
pressure of this seems to force the two middle layers together from
the start, and as the solid structure of the mesoderm apparently
belies the original hollow character of the sacs, the two sections of
the mesoderm, which are at that time divided by the lateral fold - the
dorsal segment-plates and ventral side-plates - have the appearance at
first of solid layers of cells (Figures 1.94 to 1.97). And when the
articulation of the somites begins in the sole-shaped embryonic
shield, and a couple of protovertebrae are developed in succession,
constantly increasing in number towards the rear, these cube-shaped
somites (formerly called protovertebrae, or primitive vertebrae) have
the appearance of solid dice, made up of mesodermic cells (Figure
1.93). Nevertheless, there is for a time a ventral cavity, or
provertebral cavity, even in these solid "protovertebrae" (Figure
1.143 uwh). This vesicular condition of the provertebra is of the
greatest phylogenetic interest; we must, according to the coelom
theory, regard it as an hereditary reproduction of the hollow dorsal
somites of the amphioxus (Figures 1.156 to 1.160) and the lower
vertebrates (Figures 1.161 to 1.163). This rudimentary "provertebral
cavity" has no physiological significance whatever in the
amniote-embryo; it soon disappears, being filled up with cells of the
muscular plate.

(FIGURE 1.167. Head of a shark embryo (Pristiurus), one-third of an
inch long, magnified twenty times. (From Parker.) Seen from the
ventral side.)

The innermost median part of the primitive segment plates, which lies
immediately on the chorda (Figure 1.145 ch) and the medullary tube
(m), forms the vertebral column in all the higher vertebrates (it is
wanting in the lowest); hence it may be called the skeleton plate. In
each of the provertebrae it is called the "sclerotome" (in opposition
to the outlying muscular plate, the "myotome"). From the phylogenetic
point of view the myotomes are much older than the sclerotomes. The
lower or ventral part of each sclerotome (the inner and lower edge of
the cube-shaped provertebra) divides into two plates, which grow round
the chorda, and thus form the foundation of the body of the vertebra
(wh). The upper plate presses between the chorda and the medullary
tube, the lower between the chorda and the alimentary canal (Figure
1.137 C). As the plates of two opposite provertebral pieces unite from
the right and left, a circular sheath is formed round this part of the
chorda. From this develops the BODY of a vertebra - that is to say, the
massive lower or ventral half of the bony ring, which is called the
"vertebra" proper and surrounds the medullary tube (Figures 1.164 to
1.166). The upper or dorsal half of this bony ring, the vertebral arch
(Figure 1.145 wb), arises in just the same way from the upper part of
the skeletal plate, and therefore from the inner and upper edge of the
cube-shaped primitive vertebra. As the upper edges of two opposing
somites grow together over the medullary tube from right and left, the
vertebra-arch becomes closed.

The whole of the secondary vertebra, which is thus formed from the
union of the skeletal plates of two provertebral pieces and encloses a
part of the chorda in its body, consists at first of a rather soft
mass of cells; this afterwards passes into a firmer, cartilaginous
stage, and finally into a third, permanent, bony stage. These three
stages can generally be distinguished in the greater part of the
skeleton of the higher vertebrates; at first most parts of the
skeleton are soft, tender, and membranous; they then become
cartilaginous in the course of their development, and finally bony.

(FIGURES 1.168 AND 1.169. Head of a chick embryo, of the third day.
Figure 1.168 from the front, Figure 1.169 from the right. n
rudimentary nose (olfactory pit), l rudimentary eye (optic pit,
lens-cavity), g rudimentary ear (auditory pit), v fore-brain, gl
eye-cleft. Of the three pairs of gill-arches the first has passed into
a process of the upper jaw (o) and of the lower jaw (u). (From

At the head part of the embryo in the amniotes there is not generally
a cleavage of the middle germinal layer into provertebral and lateral
plates, but the dorsal and ventral somites are blended from the first,
and form what are called the "head-plates" (Figure 1.148 k). From
these are formed the skull, the bony case of the brain, and the
muscles and corium of the body. The skull develops in the same way as
the membranous vertebral column. The right and left halves of the head
curve over the cerebral vesicle, enclose the foremost part of the
chorda below, and thus finally form a simple, soft, membranous capsule
about the brain. This is afterwards converted into a cartilaginous
primitive skull, such as we find permanently in many of the fishes.
Much later this cartilaginous skull becomes the permanent bony skull
with its various parts. The bony skull in man and all the other
amniotes is more highly differentiated and modified than that of the
lower vertebrates, the amphibia and fishes. But as the one has arisen
phylogenetically from the other, we must assume that in the former no
less than the latter the skull was originally formed from the
sclerotomes of a number of (at least nine) head-somites.

While the articulation of the vertebrate body is always obvious in the
episoma or dorsal body, and is clearly expressed in the segmentation
of the muscular plates and vertebrae, it is more latent in the
hyposoma or ventral body. Nevertheless, the hyposomites of the vegetal
half of the body are not less important than the episomites of the
animal half. The segmentation in the ventral cavity affects the
following principal systems of organs: 1, the gonads or sex-glands
(gonotomes); 2, the nephridia or kidneys (nephrotomes); and 3, the
head-gut with its gill-clefts (branchiotomes).

(FIGURE 1.170. Head of a dog embryo, seen from the front. a the two
lateral halves of the foremost cerebral vesicle, b rudimentary eye, c
middle cerebral vesicle, de first pair of gill-arches (e upper-jaw
process, d lower-jaw process), f, f apostrophe, f double apostrophe,
second, third, and fourth pairs of gill-arches, g h i k heart (g
right, h left auricle; i left, k right ventricle), l origin of the
aorta with three pairs of arches, which go to the gill-arches. (From

The metamerism of the hyposoma is less conspicuous because in all the
craniotes the cavities of the ventral segments, in the walls of which
the sexual products are developed, have long since coalesced, and
formed a single large body-cavity, owing to the disappearance of the
partition. This cenogenetic process is so old that the cavity seems to
be unsegmented from the first in all the craniotes, and the rudiment
of the gonads also is almost always unsegmented. It is the more
interesting to learn that, according to the important discovery of
Ruckert, this sexual structure is at first segmental even in the
actual selachii, and the several gonotomes only blend into a simple
sexual gland on either side secondarily.

(FIGURE 1.171. Human embryo of the fourth week (twenty-six days old),
one-fourth of an inch in length magnified twenty times, showing: point
of development of the hind-leg, umbilical cord (underneath it the
tail, bent upwards), trigeminal nerve V Trigeminus, optic-muscle nerve
III Oculo-motorius, rolling muscle nerve IV Trochlearis, rudiment of
ear (labyrinthic vesicles), pneumogastric nerve X Vagus, terminal
nerve XI Accessorius, hypoglossal nerve XII Hypoglossus, first spinal
nerve, point of development of arm (or fore-leg), true spinal nerve.
(From Moll.) The rudiments of the cerebral nerves and the roots of the
spinal nerves are especially marked. Underneath the four gill-arches
(left side) is the heart (with auricle, V and ventricle, K), under
this again the liver (L).)

Amphioxus, the sole surviving representative of the acrania, once more
yields us most interesting information; in this case the sexual glands
remain segmented throughout life. The sexually mature lancelet has, on
the right and left of the gut, a series of metamerous sacs, which are
filled with ova in the female and sperm in the male. These segmental
gonads are originally nothing else than the real gonotomes, separate
body-cavities, formed from the hyposomites of the trunk.

The gonads are the most important segmental organs of the hyposoma, in
the sense that they are phylogenetically the oldest. We find sexual
glands (as pouch-like appendages of the gastro-canal system) in most
of the lower animals, even in the medusae, etc., which have no
kidneys. The latter appear first (as a pair of excretory tubes) in the
platodes (turbellaria), and have probably been inherited from these by
the articulates (annelids) on the one hand and the unarticulated
prochordonia on the other, and from these passed to the articulated
vertebrates. The oldest form of the kidney system in this stem are the
segmental pronephridia or prorenal canals, in the same arrangement as
Boveri found them in the amphioxus. They are small canals that lie in
the frontal plane, on each side of the chorda, between the episoma and
hyposoma (Figure 1.102 n); their internal funnel-shaped opening leads
into the various body-cavities, their outer opening is the lateral
furrow of the epidermis. Originally they must have had a double
function, the carrying away of the urine from the episomites and the
release of the sexual cells from the hyposomites.

The recent investigations of Ruckert and Van Wijhe on the mesodermic
segments of the trunk and the excretory system of the selachii show
that these "primitive fishes" are closely related to the amphioxus in
this further respect. The transverse section of the shark-embryo in
Figure 1.161 shows this very clearly.

In other higher vertebrates, also, the kidneys develop (though very
differently formed later on) from similar structures, which have been
secondarily derived from the segmental pronephridia of the acrania.
The parts of the mesoderm at which the first traces of them are found
are usually called the middle or mesenteric plates. As the first
traces of the gonads make their appearance in the lining of these
middle plates nearer inward (or the middle) from the inner funnels of
the nephro-canals, it is better to count this part of the mesoderm
with the hyposoma.

The chief and oldest organ of the vertebrate hyposoma, the alimentary
canal, is generally described as an unsegmented organ. But we could
just as well say that it is the oldest of all the segmented organs of
the vertebrate; the double row of the coelom-pouches grows out of the
dorsal wall of the gut, on either side of the chorda. In the brief
period during which these segmental coelom-pouches are still openly
connected with the gut, they look just like a double chain of
segmented visceral glands. But apart from this, we have originally in
all vertebrates an important articulation of the fore-gut, that is
wanting in the lower gut, the segmentation of the branchial (gill)

(FIGURE 1.172. Transverse section of the shoulder and fore-limb (wing)
of a chick-embryo of the fourth day, magnified about twenty times.
Beside the medullary tube we can see on each side three clear streaks
in the dark dorsal wall, which advance into the rudimentary fore-limb
or wing (e). The uppermost of them is the muscular plate; the middle
is the hind and the lowest the fore root of a spinal nerve. Under the
chorda in the middle is the single aorta, at each side of it a
cardinal vein, and below these the primitive kidneys. The gut is
almost closed. The ventral wall advances into the amnion, which
encloses the embryo. (From Remak.)

FIGURE 1.173. Transverse section of the pelvic region and hind legs of
a chick-embryo of the fourth day, magnified about forty times. h
horn-plate, w medullary tube, n canal of the tube, u primitive
kidneys, x chorda, e hind legs, b allantoic canal in the ventral wall,
t aorta, v cardinal veins, a gut, d gut-gland layer, f gut-fibre
layer, g embryonic epithelium, r dorsal muscles, c body-cavity or
coeloma. (From Waldeyer.))

The gill-clefts, which originally in the older acrania pierced the
wall of the fore-gut, and the gill-arches that separated them, were
presumably also segmental, and distributed among the various metamera
of the chain, like the gonads in the after-gut and the nephridia. In
the amphioxus, too, they are still segmentally formed. Probably there
was a division of labour of the hyposomites in the older (and long
extinct) acrania, in such wise that those of the fore-gut took over
the function of breathing and those of the after-gut that of
reproduction. The former developed into gill-pouches, the latter into
sex-pouches. There may have been primitive kidneys in both. Though the
gills have lost their function in the higher animals, certain parts of
them have been generally maintained in the embryo by a tenacious
heredity. At a very early stage we notice in the embryo of man and the
other amniotes, at each side of the head, the remarkable and important
structures which we call the gill-arches and gill-clefts (Figures
1.167 to 1.170 f). They belong to the characteristic and inalienable
organs of the amniote-embryo, and are found always in the same spot
and with the same arrangement and structure. There are formed to the
right and left in the lateral wall of the fore-gut cavity, in its
foremost part, first a pair and then several pairs of sac-shaped
inlets, that pierce the whole thickness of the lateral wall of the
head. They are thus converted into clefts, through which one can
penetrate freely from without into the gullet. The wall thickens
between these branchial folds, and changes into an arch-like or
sickle-shaped piece - the gill, or gullet-arch. In this the muscles and
skeletal parts of the branchial gut separate; a blood-vessel arch
rises afterwards on their inner side (Figure 1.98 ka). The number of

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