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Ernst Heinrich Philipp August Haeckel.

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internal organisation, I omit all the subordinate points, and restrict
myself to the most important characteristics.

Much, of course, will seem to the reader to be essential that is only
of subordinate and secondary interest, or even not essential at all,
in the light of comparative anatomy and embryology. For instance, the
skull and vertebral column and the extremities are non-essential in
this sense. It is true that these parts are very important
PHYSIOLOGICALLY; but for the MORPHOLOGICAL conception of the
vertebrate they are not essential, because they are only found in the
higher, not the lower, vertebrates. The lowest vertebrates have
neither skull nor vertebrae, and no extremities or limbs. Even the
human embryo passes through a stage in which it has no skull or
vertebrae; the trunk is quite simple, and there is yet no trace of
arms and legs. At this stage of development man, like every other
higher vertebrate, is essentially similar to the simplest vertebrate
form, which we now find in only one living specimen. This one lowest
vertebrate that merits the closest study - undoubtedly the most
interesting of all the vertebrates after man - is the famous lancelet
or amphioxus, to which we have already often referred. As we are going
to study it more closely later on (Chapters 2.16 and 2.17), I will
only make one or two passing observations on it here.

The amphioxus lives buried in the sand of the sea, is about one or two
inches in length, and has, when fully developed, the shape of a very
simple, longish, lancet-like leaf; hence its name of the lancelet. The
narrow body is compressed on both sides, almost equally pointed at the
fore and hind ends, without any trace of external appendages or
articulation of the body into head, neck, breast, abdomen, etc. Its
whole shape is so simple that its first discoverer thought it was a
naked snail. It was not until much later - half a century ago - that the
tiny creature was studied more carefully, and was found to be a true
vertebrate. More recent investigations have shown that it is of the
greatest importance in connection with the comparative anatomy and
ontogeny of the vertebrates, and therefore with human phylogeny. The
amphioxus reveals the great secret of the origin of the vertebrates
from the invertebrate vermalia, and in its development and structure
connects directly with certain lower tunicates, the ascidia.

When we make a number of sections of the body of the amphioxus,
firstly vertical longitudinal sections through the whole body from end
to end, and secondly transverse sections from right to left, we get
anatomic pictures of the utmost instructiveness (cf. Figures 1.98 to
1.102). In the main they correspond to the ideal which we form, with
the aid of comparative anatomy and ontogeny, of the primitive type or
build of the vertebrate - the long-extinct form to which the whole stem
owes its origin. As we take the phylogenetic unity of the vertebrate
stem to be beyond dispute, and assume a common origin from a primitive
stem-form for all the vertebrates, from amphioxus to man, we are
justified in forming a definite morphological idea of this primitive
vertebrate (Prospondylus or Vertebraea). We need only imagine a few
slight and unessential changes in the real sections of the amphioxus
in order to have this ideal anatomic figure or diagram of the
primitive vertebrate form, as we see in Figures 1.98 to 1.102. The
amphioxus departs so little from this primitive form that we may, in a
certain sense, describe it as a modified "primitive vertebrate."* (*
The ideal figure of the vertebrate as given in Figures 1.98 to 1.102
is a hypothetical scheme or diagram, that has been chiefly constructed
on the lines of the amphioxus, but with a certain attention to the
comparative anatomy and ontogeny of the ascidia and appendicularia on
the one hand, and of the cyclostoma and selachii on the other. This
diagram has no pretension whatever to be an "exact picture," but
merely an attempt to reconstruct hypothetically the unknown and long
extinct vertebrate stem-form, an ideal "archetype.")

The outer form of our hypothetical primitive vertebrate was at all
events very simple, and probably more or less similar to that of the
lancelet. The bilateral or bilateral-symmetrical body is stretched out
lengthways and compressed at the sides (Figures 1.98 to 1.100), oval
in section (Figures 1.101 and 1.102). There are no external
articulation and no external appendages, in the shape of limbs, legs,
or fins. On the other hand, the division of the body into two
sections, head and trunk, was probably clearer in Prospondylus than it
is in its little-changed ancestor, the amphioxus. In both animals the
fore or head-half of the body contains different organs from the
trunk, and different on the dorsal from on the ventral side. As this
important division is found even in the sea-squirt, the remarkable
invertebrate stem-relative of the vertebrates, we may assume that it
was also found in the prochordonia, the common ancestors of both
stems. It is also very pronounced in the young larvae of the
cyclostoma; this fact is particularly interesting, as this
palingenetic larva-form is in other respects also an important
connecting-link between the higher vertebrates and the acrania.

(FIGURES 1.98 TO 1.102. The ideal primitive vertebrate (prospondylus).
Diagram. Figure 1.98 side-view (from the left). Figure 1.99 back-view.
Figure 1.100 front view. Figure 1.101 transverse section through the
head (to the left through the gill-pouches, to the right through the
gill-clefts). Figure 1.102 transverse section of the trunk (to the
right a pro-renal canal is affected). a aorta, af anus, au eye, b
lateral furrow (primitive renal process), c coeloma (body-cavity), d
small intestine, e parietal eye (epiphysis), f fin border of the skin,
g auditory vesicle, gh brain, h heart, i muscular cavity (dorsal
coelom-pouch), k gill-grut, ka gill-artery, kg gill-arch, ks
gill-folds, l liver, ma stomach, md mouth, ms muscles, na nose (smell
pit), n renal canals, u apertures of same, o outer skin, p gullet, r
spinal marrow, a sexual glands (gonads), t corium, u kidney-openings
(pores of the lateral furrow), v visceral vein (chief vein). x chorda,
y hypophysis (urinary appendage), z gullet-groove or gill-groove
(hypobranchial groove).)

The head of the acrania, or the anterior half of the body (both of the
real amphioxus and the ideal prospondylus), contains the branchial
(gill) gut and heart in the ventral section and the brain and
sense-organs in the dorsal section. The trunk, or posterior half of
the body, contains the hepatic (liver) gut and sexual-glands in the
ventral part, and the spinal marrow and most of the muscles in the
dorsal part.

In the longitudinal section of the ideal vertebrate (Figure 1.98) we
have in the middle of the body a thin and flexible, but stiff,
cylindrical rod, pointed at both ends (ch). It goes the whole length
through the middle of the body, and forms, as the central skeletal
axis, the original structure of the later vertebral column. This is
the axial rod, or chorda dorsalis, also called chorda vertebralis,
vertebral cord, axial cord, dorsal cord, notochorda, or, briefly,
chorda. This solid, but flexible and elastic, axial rod consists of a
cartilaginous mass of cells, and forms the inner axial skeleton or
central frame of the body; it is only found in vertebrates and
tunicates, not in any other animals. As the first structure of the
spinal column it has the same radical significance in all vertebrates,
from the amphioxus to man. But it is only in the amphioxus and the
cyclostoma that the axial rod retains its simplest form throughout
life. In man and all the higher vertebrates it is found only in the
earlier embryonic period, and is afterwards replaced by the
articulated vertebral column.

The axial rod or chorda is the real solid chief axis of the vertebrate
body, and at the same time corresponds to the ideal long-axis, and
serves to direct us with some confidence in the orientation of the
principal organs. We therefore take the vertebrate-body in its
original, natural disposition, in which the long-axis lies
horizontally, the dorsal side upward and the ventral side downward
(Figure 1.98). When we make a vertical section through the whole
length of this long axis, the body divides into two equal and
symmetrical halves, right and left. In each half we have ORIGINALLY
the same organs in the same disposition and connection; only their
disposal in relation to the vertical plane of section, or median
plane, is exactly reversed: the left half is the reflection of the
right. We call the two halves antimera (opposed-parts). In the
vertical plane of section that divides the two halves the sagittal
("arrow") axis, or "dorsoventral axis," goes from the back to the
belly, corresponding to the sagittal seam of the skull. But when we
make a horizontal longitudinal section through the chorda, the whole
body divides into a dorsal and a ventral half. The line of section
that passes through the body from right to left is the transverse,
frontal, or lateral axis.

The two halves of the vertebrate body that are separated by this
horizontal transverse axis and by the chorda have quite different
characters. The dorsal half is mainly the animal part of the body, and
contains the greater part of what are called the animal organs, the
nervous system, muscular system, osseous system, etc. - the instruments
of movement and sensation. The ventral half is essentially the
vegetative half of the body, and contains the greater part of the
vertebrate's vegetal organs, the visceral and vascular systems, sexual
system, etc. - the instruments of nutrition and reproduction. Hence in
the construction of the dorsal half it is chiefly the outer, and in
the construction of the ventral half chiefly the inner, germinal layer
that is engaged. Each of the two halves develops in the shape of a
tube, and encloses a cavity in which another tube is found. The dorsal
half contains the narrow spinal-column cavity or vertebral canal ABOVE
the chorda, in which lies the tube-shaped central nervous system, the
medullary tube. The ventral half contains the much more spacious
visceral cavity or body-cavity UNDERNEATH the chorda, in which we find
the alimentary canal and all its appendages.

The medullary tube, as the central nervous system or psychic organ of
the vertebrate is called in its first stage, consists, in man and all
the higher vertebrates, of two different parts: the large brain,
contained in the skull, and the long spinal cord which stretches from
there over the whole dorsal part of the trunk. Even in the primitive
vertebrate this composition is plainly indicated. The fore half of the
body, which corresponds to the head, encloses a knob-shaped vesicle,
the brain (gh); this is prolonged backwards into the thin cylindrical
tube of the spinal marrow (r). Hence we find here this very important
psychic organ, which accomplishes sensation, will, and thought, in the
vertebrates, in its simplest form. The thick wall of the nerve-tube,
which runs through the long axis of the body immediately over the
axial rod, encloses a narrow central canal filled with fluid (Figures
1.98 to 1.102 r). We still find the medullary tube in this very simple
form for a time in the embryo of all the vertebrates, and it retains
this form in the amphioxus throughout life; only in the latter case
the cylindrical medullary tube barely indicates the separation of
brain and spinal cord. The lancelet's medullary tube runs nearly the
whole length of the body, above the chorda, in the shape of a long
thin tube of almost equal diameter throughout, and there is only a
slight swelling of it right at the front to represent the rudiment of
a cerebral lobe. It is probable that this peculiarity of the amphioxus
is connected with the partial atrophy of its head, as the ascidian
larvae on the one hand and the young cyclostoma on the other clearly
show a division of the vesicular brain, or head marrow, from the
thinner, tubular spinal marrow.

Probably we must trace to the same phylogenetic cause the defective
nature of the sense organs of the amphioxus, which we will describe
later (Chapter 2.16). Prospondylus, on the other hand, probably had
three pairs of sense-organs, though of a simple character, a pair of,
or a single olfactory depression, right in front (Figures 1.98 and
1.99, na), a pair of eyes (au) in the lateral walls of the brain, and
a pair of simple auscultory vesicles (g) behind. There was also,
perhaps, a single parietal or "pineal" eye at the top of the skull
(epiphysis, e).

In the vertical median plane (or middle plane, dividing the bilateral
body into right and left halves) we have in the acrania, underneath
the chorda, the mesentery and visceral tube, and above it the
medullary tube; and above the latter a membranous partition of the two
halves of the body. With this partition is connected the mass of
connective tissue which acts as a sheath both for the medullary tube
and the underlying chorda, and is, therefore, called the chord-sheath
(perichorda); it originates from the dorsal and median part of the
coelom-pouches, which we shall call the skeleton plate or "sclerotom"
in the craniote embryo. In the latter the chief part of the
skeleton - the vertebral column and skull - develops from this
chord-sheath; in the acrania it retains its simple form as a soft
connective matter, from which are formed the membranous partitions
between the various muscular plates or myotomes (Figures 1.98 and 1.99
ms).

To the right and left of the cord-sheath, at each side of the
medullary tube and the underlying axial rod, we find in all the
vertebrates the large masses of muscle that constitute the musculature
of the trunk and effect its movements. Although these are very
elaborately differentiated and connected in the developed vertebrate
(corresponding to the various parts of the bony skeleton), in our
ideal primitive vertebrate we can distinguish only two pairs of these
principal muscles, which run the whole length of the body parallel to
the chorda. These are the upper (dorsal) and lower (ventral) lateral
muscles of the trunk. The upper (dorsal) muscles, or the original
dorsal muscles (Figure 1.102 ms), form the thick mass of flesh on the
back. The lower (ventral) muscles, or the original muscles of the
belly, form the fleshy wall of the abdomen. Both sets are segmented,
and consist of a double row of muscular plates (Figures 1.98 and 1.99
ms); the number of these myotomes determines the number of joints in
the trunk, or metamera. The myotomes are also developed from the thick
wall of the coelom-pouches (Figure 1.102 i).

Outside this muscular tube we have the external envelope of the
vertebrate body, which is known as the corium or cutis. This strong
and thick envelope consists, in its deeper strata, chiefly of fat and
loose connective tissue, and in its upper layers of cutaneous muscles
and firmer connective tissue. It covers the whole surface of the
fleshy body, and is of considerable thickness in all the craniota. But
in the acrania the corium is merely a thin plate of connective tissue,
an insignificant "corium-plate" (lamella corii, Figures 1.98 to 1.102
t).

Immediately above the corium is the outer skin (epidermis, o), the
general covering of the whole outer surface. In the higher vertebrates
the hairs, nails, feathers, claws, scales, etc., grow out of this
epidermis. It consists, with all its appendages and products, of
simple cells, and has no blood-vessels. Its cells are connected with
the terminations of the sensory nerves. Originally, the outer skin is
a perfectly simple covering of the outer surface of the body, composed
only of homogeneous cells - a permanent horn-plate. In this simplest
form, as a one-layered epithelium, we find it, at first, in all the
vertebrates, and throughout life in the acrania. It afterwards grows
thicker in the higher vertebrates, and divides into two strata - an
outer, firmer corneous (horn) layer and an inner, softer mucus-layer;
also a number of external and internal appendages grow out of it:
outwardly, the hairs, nails, claws, etc., and inwardly, the
sweat-glands, fat-glands, etc.

It is probable that in our primitive vertebrate the skin was raised in
the middle line of the body in the shape of a vertical fin border (f).
A similar fringe, going round the greater part of the body, is found
to-day in the amphioxus and the cyclostoma; we also find one in the
tail of fish-larvae and tadpoles.

Now that we have considered the external parts of the vertebrate and
the animal organs, which mainly lie in the dorsal half, above the
chorda, we turn to the vegetal organs, which lie for the most part in
the ventral half, below the axial rod. Here we find a large
body-cavity or visceral cavity in all the craniota. The spacious
cavity that encloses the greater part of the viscera corresponds to
only a part of the original coeloma, which we considered in Chapter
1.10; hence it nay be called the metacoeloma. As a rule, it is still
briefly called the coeloma; formerly it was known in anatomy as the
pleuroperitoneal cavity. In man and the other mammals (but only in
these) this coeloma divides, when fully developed, into two different
cavities, which are separated by a transverse partition - the muscular
diaphragm. The fore or pectoral cavity (pleura-cavity) contains the
oesophagus (gullet), heart, and lungs; the hind or peritoneal or
abdominal cavity contains the stomach, small and large intestines,
liver, pancreas, kidneys, etc. But in the vertebrate embryo, before
the diaphragm is developed, the two cavities form a single continuous
body-cavity, and we find it thus in all the lower vertebrates
throughout life. This body-cavity is clothed with a delicate layer of
cells, the coelom-epithelium. In the acrania the coelom is segmented
both dorsally and ventrally, as their muscular pouches and primitive
genital organs plainly show (Figure 1.102).

The chief of the viscera in the body-cavity is the alimentary canal,
the organ that represents the whole body in the gastrula. In all the
vertebrates it is a long tube, enclosed in the body-cavity and more or
less differentiated in length, and has two apertures - a mouth for
taking in food (Figures 1.98 and 1.100 md) and an anus for the
ejection of unusable matter or excrements (af). With the alimentary
canal a number of glands are connected which are of great importance
for the vertebrate body, and which all grow out of the canal. Glands
of this kind are the salivary glands, the lungs, the liver, and many
smaller glands. Nearly all these glands are wanting in the acrania;
probably there were merely a couple of simple hepatic tubes (Figures
1.98 and 1.100 l) in the vertebrate stem-form. The wall of the
alimentary canal and all its appendages consists of two different
layers; the inner, cellular clothing is the gut-gland-layer, and the
outer, fibrous envelope consists of the gut-fibre-layer; it is mainly
composed of muscular fibres which accomplish the digestive movements
of the canal, and of connective-tissue fibres that form a firm
envelope. We have a continuation of it in the mesentery, a thin,
bandage-like layer, by means of which the alimentary canal is fastened
to the ventral side of the chorda, originally the dorsal partition of
the two coelom-pouches. The alimentary canal is variously modified in
the vertebrates both as a whole and in its several sections, though
the original structure is always the same, and is very simple. As a
rule, it is longer (often several times longer) than the body, and
therefore folded and winding within the body-cavity, especially at the
lower end. In man and the higher vertebrates it is divided into
several sections, often separated by valves - the mouth, pharynx,
oesophagus, stomach, small and large intestine, and rectum. All these
parts develop from a very simple structure, which originally
(throughout life in the amphioxus) runs from end to end under the
chorda in the shape of a straight cylindrical canal.

As the alimentary canal may be regarded morphologically as the oldest
and most important organ in the body, it is interesting to understand
its essential features in the vertebrate more fully, and distinguish
them from unessential features. In this connection we must
particularly note that the alimentary canal of every vertebrate shows
a very characteristic division into two sections - a fore and a hind
chamber. The fore chamber is the head-gut or branchial gut (Figures
1.98 to 1.100 p, k), and is chiefly occupied with respiration. The
hind section is the trunk-gut or hepatic gut, which accomplishes
digestion (ma, d). In all vertebrates there are formed, at an early
stage, to the right and left in the fore-part of the head-gut, certain
special clefts that have an intimate connection with the original
respiratory apparatus of the vertebrate - the branchial (gill) clefts
(ks). All the lower vertebrates, the lancelets, lampreys, and fishes,
are constantly taking in water at the mouth, and letting it out again
by the lateral clefts of the gullet. This water serves for breathing.
The oxygen contained in it is inspired by the blood-canals, which
spread out on the parts between the gill-clefts, the gill-arches (kg).
These very characteristic branchial clefts and arches are found in the
embryo of man and all the higher vertebrates at an early stage of
development, just as we find them throughout life in the lower
vertebrates. However, these clefts and arches never act as respiratory
organs in the mammals, birds, and reptiles, but gradually develop into
quite different parts. Still, the fact that they are found at first in
the same form as in the fishes is one of the most interesting proofs
of the descent of these three higher classes from the fishes.

Not less interesting and important is an organ that develops from the
ventral wall in all vertebrates - the gill-groove or hypobranchial
groove. In the acrania and the ascidiae it consists throughout life of
a glandular ciliated groove, which runs down from the mouth in the
ventral middle line of the gill-gut, and takes small particles of food
to the stomach (Figure 1.101 z). But in the craniota the thyroid gland
(thyreoidea) is developed from it, the gland that lies in front of the
larynx, and which, when pathologically enlarged, forms goitre
(struma).

From the head-gut we get not only the gills, the organs of
water-breathing in the lower vertebrates, but also the lungs, the
organs of atmospheric breathing in the five higher classes. In these
cases a vesicular fold appears in the gullet of the embryo at an early
stage, and gradually takes the shape of two spacious sacs, which are
afterwards filled with air. These sacs are the two air-breathing
lungs, which take the place of the water-breathing gills. But the
vesicular invagination, from which the lungs arise, is merely the
familiar air-filled vesicle, which we call the floating-bladder of the
fish, and which alters its specific weight, acting as hydrostatic
organ or floating apparatus. This structure is not found in the lowest
vertebrate classes - the acrania and cyclostoma. We shall see more of
it in Volume 2.

The second chief section of the vertebrate-gut, the trunk or
liver-gut, which accomplishes digestion, is of very simple
construction in the acrania. It consists of two different chambers.
The first chamber, immediately behind the gill-gut, is the expanded
stomach (ma); the second, narrower and longer chamber, is the straight
small intestine (d): it issues behind on the ventral side by the anus
(af). Near the limit of the two chambers in the visceral cavity we
find the liver, in the shape of a simple tube or blind sac (l); in the
amphioxus it is single; in the prospondylus it was probably double
(Figures 1.98 and 1.100 l).

Closely related morphologically and physiologically to the alimentary
canal is the vascular system of the vertebrate, the chief sections of
which develop from the fibrous gut-layer. It consists of two different
but directly connected parts, the system of blood-vessels and that of
lymph-vessels. In the passages of the one we find red blood, and in
the other colourless lymph. To the lymphatic system belong, first of
all, the lymphatic canals proper or absorbent veins, which are
distributed among all the organs, and absorb the used-up juices from
the tissues, and conduct them into the venous blood; but besides these
there are the chyle-vessels, which absorb the white chyle, the milky
fluid prepared by the alimentary canal from the food, and conduct this


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