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blast; but although the nervous centres are thus formed, and though the cranial nerves arise also as small invaginations of epiblast, yet the mass of nerves which permeate the body in all directions cannot be so derived, but must originate from mesoblastic tissue. The ganglionic cells or nerve corpuscles are doubtless derived from embryonic cells, which became transformed in nature and send forth processes (very generally) in the way described.*

As to the nerve-fibres, they seem to be formed by the coalescence

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Fig. 151.-LONGITUDINAL SECTION OF AN EMBRYO, SHOWING THE INCIPIENT VERTEBRAL
COLUMN WITH ITS VERTEBRAL CENTRA AND SPINOUS PROCESSES.

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cl. The cloaca.

v. The urinary bladder and pedicle of the allan-
tois.

u, u'. The umbilicus, or root of the umbilical
cord, containing the duct from the intestine
to the yelk-sac, the pedicle of the allantois
(or urachus), and the umbilical vessels.
w. The Wolffian body, which appears as an
elongated viscus, with many transverse lines
situated above and between the two letters,
i and i; whereof the left-hand i is placed
upon the stomach, and the right-hand i on
the large intestine close to the cecum and
entrance of the small intestine.

in linear series of spindle-shaped cells, and to be at first of the nature of pale or grey fibres, but afterwards, in great part, to acquire a medullary sheath and to become white fibres.

§ 8. We may next consider the origin of the organs, and first the internal skeleton.

The AXIAL SKELETON makes its appearance much earlier than the appendicular skeleton, inasmuch as its foundations are laid in the lamina dorsales bounding the medullary groove, and in the notochord as already described. The distinction of the axial skeleton into its vertebral and cranial portions is laid down from a very early period, since the enlargement of the anterior end of the medullary groove into the cerebral vesicles at once marks out the cranial part, a distinction rendered yet plainer by the non-extension of the chorda dorsalis forwards through it.

See ante, p. 255.

The earliest indication of the segmented condition of the VERTEBRAL SKELETON is the appearance of the quadrate masses of tissue, appearing serially in pairs behind the head-the so-called proto-vertebræ (Fig. 146, pr), or dorsal segments, already noticed as arising in the mesoblast on each side of the chorda dorsalis and medullary groove. The front part of the first of these bodies corresponds in position with the atlas, and each pair of dorsal segments gives rise, amongst other structures, to part of the bony spine. The dorsal segments do not, however, correspond with the future vertébræ, but each segment becomes transformed into: (1) the hinder part of one vertebra, (2) the anterior part of the vertebra next behind, (3) the roots of a spinal nerve, and (4) the muscles and skin immediately connected with the vertebral parts so formed. Thus each primordial vertebra becomes ultimately segmented, while each successive pair of such primordial vertebrae ultimately coalesce, and so a different segmentation is brought about from that which appears at first; the points of separation of the later segmentation, alternating with those of the earlier segmentation. The first dorsal segment of each side differs from those which succeed, in that when it becomes segmented, its anterior half has nothing in front wherewith to coalesce, and thus the atlas must be formed from half a primordial vertebra instead of being formed, like the other vertebræ, from the hinder half of one primordial vertebra and the anterior half of the primordial vertebra next behind.

Moreover, while the upper part of each dorsal plate becomes a segment of dorsal muscle, with its skin, the lower part undergoes a different change in its anterior (cephalic) and posterior halves. Anteriorly it gives rise to the root and ganglion of a spinal nerve. Posteriorly it gives rise to the transverse process of a vertebra or proximal portion of a rib.

The inner part of each dorsal plate bifurcates as it extends inwards. One branch ascends in the dorsal lamina till it meets its fellow of the opposite side and so forms the foundation of the future neural arch. The other branch advances inwards above and below the chorda, and blends with its fellow of the opposite side to form the foundation of the central portion of a vertebral segment. Thus each permanent vertebra is the offspring of the parts of two adjacent primordial vertebra. Its neural arch, transverse processes, the proximal part of its ribs, and the cephalic portion of its centrum, are formed from the hinder end of one proto-vertebra, while the rest of its centrum and its spinal nerves are formed from the anterior (cephalic) end of the proto-vertebra next behind.

This condition having been, as it were, laid down in soft tissue, transformations of parts of the structures thus formed, into cartilage, soon begin.

Cartilage of the ordinary kind first invades the body of each vertebra and surrounds the chorda, encroaching on and constricting it at intervals, the chorda yet continuing a structure of large thinwalled cells enclosed in a fibrous sheath. Cartilage becomes also

deposited at intervals along each dorsal lamina, forming cartilaginous neural arches which, however, do not for some time fully unite together on the dorsal side of the myelon. Cartilages also extend down in each ventral lamina as the cartilaginous predecessors of the ribs, and those of the thorax, by their median fusion in the mid-ventral line, lay the foundation of the sternum.

The third stage of vertebral development, or the ossification of the spinal column, begins to show itself as three (or four) ossific centres arising in each of the vertebræ. These centres are placed one in each lateral part (at the junction of the transverse process and neural arch), and the other, or third, in the centrum. This last is sometimes double at first.

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Besides these separate centres each vertebra, while immature, has certain epiphyses or temporary separate terminal ossifications. Thus the tip of each prominent process (neural spines, transverse and articular processes and the metapophyses) has its epiphysis, and a thin lamina of bone is developed as an epiphysis on both the anterior and posterior surface of each centrum. The atlas ossifies from two lateral ossifications, and one median one ventrally placed.

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In the axis, in addition to the ordinary ossific centres found in other vertebræ, the odontoid process ossifies from two centres, placed side by side, which soon unite. There is also an epiphysis at the apex of the odontoid process, and one between it and the centrum of the axis as well as on the hinder surface of the latter. Thus the odontoid process ossifies as if it were, as in fact it is, the true centrum of the atlas vertebra.

The ribs are ossified each from one centre, with an epiphysis for the tuberculum and another for the capitulum.

In the cervical vertebra more or fewer of the transverse processes ossify, at least occasionally, from a distinct ossific centre in their ventral branch, a circumstance which tends to show (what is in fact the case,) that these double (or perforated) transverse processes are ribs with very short bodies.

Similarly the sacral vertebræ have each a distinct ossific centre in each of their lateral masses.

The manubrium ossifies after the other segments of the sternum, which seem each to ossify from one centre.

§ 9. The DEVELOPMENT OF THE SKULL takes place in a specially circuitous manner, so that its early stages are strikingly unlike its mature condition. The first indication of the future skull is given by the expansion, before mentioned, of the anterior end of the medullary groove, which expansion, as has been said, becomes divided by two lateral constrictions (one in front of the other, on each side) so that three rounded vesicles are formed lying serially one before the other. The notochord extends forwards to beneath the second of these vesicles, which bend down sharply in front of its anterior termination, so that there comes to be one vesicle above, one in front of, and one below the anterior termination of the chorda.

These vesicles are, as we shall hereafter see, the commencements of the future brain.

In the walls of the ascending lamina dorsales, which bound the vesicles laterally, there are no quadrate thickenings like those developed on each side of the chorda in the vertebral region, while peculiar developments take place in their ventral laminæ. For while the medullary groove is being arched over and converted into the great axial, neural canal, by the ascending lamina dorsales, another axial canal is being formed beneath the neural one by the descending lamina ventrales. This second axial canal is the rudimentary alimentary one. The ventral laminæ, as they bend down to enclose the incipient pharynx, grow thinner and thinner at successive intervals, one behind the other, till a series of perforations, the visceral clefts, are formed, one after the other, each cleft leading from the exterior into the pharyngeal cavity. Four such visceral clefts appear on each side. The perforation proceeds from within outwards, the hypoblast being absorbed first, then the inner part of the mesoblast, and finally, the whole of the mesoblast, the hypoblast growing outwards along each advancing wall of each aperture, and ultimately becoming continuous with the epiblast. In front of each cleft the wall of the ventral lamina becomes more or less thickened, forming what are called the visceral arches (Fig. 152, 1, 2, 3, +)-each such lateral series of arches being at first separate from their fellows of the opposite side, as are the ventral lamina themselves. Meantime another pair of vesicles-the cerebral vesicles-grow forwards (side by side) from that which was at first the most anterior vesicle; and a pit formed beneath the outer anterior part of each cerebral vesicle lays the foundation of the future nasal organs (Fig. 152, na), while two other and more posterior invaginations on each side (one beneath what was the first vesicle and the other beside the hindmost) respectively lay the foundations of the eye and of the ear. mouth is formed by a superficial depression-as will be subsequently more fully explained. On each side of the mouth the first visceral arch has meantime grown down and united distally with its fellow of

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and lungs grow out from that canal ventrally. The renal and sexual organs arise close to the bifurcation of the ventral laminæ into the somatopleures and splanchnopleures, and the limbs bud forth as rounded processes, the distal ends of which subsequently grow out into digits, while the limbs themselves become flexed in reverse directions. On

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each side of the body, close behind the head, certain apertures appear, which lead from the exterior to what becomes the pharynx. These openings are the visceral clefts, and their interspaces are the visceral arches. Almost all the clefts disappear before birth. The formation of these various organs will be detailed subsequently, but their relations to the three primary layers, from which the whole of them are built up, may be summarized as follows. The epiblast gives rise to the epidermis of the skin, the nerVous centres, and the organs of sense. The hypoblast forms the epithelium of the alimentary canal, except its two ends, and of the glands which open into it. The mesoblast forms the internal skeleton, the muscles, connective tissue, peritoneum and pleuræ, and the vascular and secreting organs generally. It forms, therefore, the great bulk of the cat's body.

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Fig. 149.-SECTION THROUGH CORNU OF UTERUS,

SHOWING THE TWO POINTS OF ATTACHMENT

OF THE ZONARY PLACENTA, THE EMBRYO
BEING REMOVED.

pl. Placenta.

m. Its maternal portion sending processes between.

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Tufts projecting from chorion or membrane, enveloping embryo.

rc. The umbilical cord.

am. Amnion.

ut, ut. Walls of the uterus.

§ 6. During the whole process of development the germ is nourished by absorption. Within the Graafian follicle it profits by the cells of the discus proligerus and membrana granulosa, and when cast forth from its follicle into the cavity of the uterus it absorbs nutriment from the secretions of the uterine walls by processes, or villi, which grow forth on all sides from the surface of its chorion. It also feeds upon the contents of the umbilical vesicle, absorbing nutriment thence by the help of the vessels which there circulate, and which are at first of great relative size and importance. With the development of the allantois, however, a new condition obtains. That organ is destined to convey out embryonic blood-vessels to the surface of the ovum, so that they may there be placed in intimate relation with the blood-vessels of a special, corresponding maternal structure, which is formed in the wall of the uterus around the circumference of the therein-contained ovum.

While the ovum is undergoing the incipient stages of development, corresponding changes take place in the maternal structures. The presence of the impregnated ovum within the uterus is accompanied by the growth, on the inner surface of that organ, of a

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