The Vertebrate Endoskeleton
This consists of connective tissue, to which cartilage and bone may be added in various proportions; together with the tissue of the notochord and its sheath, which cannot be classed under either of those heads. The endoskeleton is distinguishable into two independant portions- the one axial, or belonging to the head and trunk; the other, appendicular, to the limbs.The axial endosheleton usually consists of two systems of skeletal parts, the spinal system, and the cranial system, the distinction between which arises in the following way in the higher Vertebrata:
The primitive groove is, at first, a simple straight depression, of equal diameter throughout; but, as its sides rise and the dorsal laminae gradually close over (this process commencing in the anterior moiety of their length, in the future cephalic region), the one part becomes wider than the other, and indicates the cephalic region (Fig. 4, A). The notochord. which underlies the groove, terminates in a point at a little distance behind the anterior end of the cephalic enlargement, and indeed under the median of three dilatations which it presents. So much of the floor of the enlarge ment as lies in front of the end of the notochord, bends down at right angles to the rest; so that the anterior enlargement, or anterior cerebral vesicle, as it is now called, lies in front of the end of the notochord; the median enlargement, or the middle cerebral vesicle, above its extremity; and the hin-der enlargement, or the posterior cerebral vesicle, behind that extremity (Fig. 4, D and E).
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| Fig. 4. - Successive stages of the development of the head of a Chick. I, II, III, first, second, and third cerebral vesicles; Ia, vesicle of the cerebral hemisphere; Ib, vesicle of the third ventricle; a, rudiments of the eyes and optic nerves; b, of the ears; g of the olfactory organs; d, the infundibulum; e, the pineal gland; c, protovertebrae; h. notochord; 1, 2, 8, 4, 5, visceral arches ; V, VII, VIII, the trigeminal portio dura, and ninth and tenth pairs of cranial nerves; the nasal process; l, the maxillary process; ec, tlie first visceral clelt. A, B, upper and under views of the head of a Chick at the end of the second day. 0, side-view at the third day. D. side-view at seventy-five hours. 33, aide-view of the head of a Chick at the fifth day, which has been subjected to slight pressxire. F, head of a Chick at the sixth day, viewed from below. |
Behind, the posterior cerebral vesicle passes into the primitively tubular spinal cord (Fig. 4, A). Where it does so, the head ends, and the spinal column begins; but no line of demarcation is at first visible between these two, the indifl'erent tissues which ensheath the notochord passing without interruption from one region to the other, and retaining the same character throughout.
The first essential differentiation between the skull and the vertebral column is efiected by the appearance of the protovertebrce. At regular intervals, commencing at the anterior part of the cervical region, and gradually extending backward, the indifferent tissue on each side of the notochord undergoes a histological change, and gives rise to more opaque, quadrate masses, on opposite sides of the notochord (Fig. 2, B, C). Each pair of these gradually unite above and below that structure, and send arched prolongations into the walls of the spinal canal, so as to constitute a protovertebra.
No protovertebrse appear in the floor of the skull, so that, even in this early stage, a clear distinction is drawn between the skull and the spinal column.
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