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
to the limbs.
The axial endosheleton
usually consists of two systems of
skeletal parts, the spinal system
, and the cranial system
distinction between which arises in the following way in the
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
|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.
The under surface of the anterior vesicle lies in a
kind of pit, in front of, and rather below, the apex of the notochord,
and the pituitary gland
is developed in connection with
it. From the opposite upper surface of the same vesicle the pineal gland
is evolved, and the part of the anterior cerebral
vesicle in connection with which these remarkable bodies arise,
is the future third ventricle
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
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.