THE VERTEBRATE ENDOSKELETON. 15 



skeletal parts, the sjnnal 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 depres- 

 sion, of equal diameter throughout ; but, as its sides rise and 

 the dorsal laminae gradually close over (this process commen- 

 cing in the anterior moiety of their length, in the future ce- 

 phalic 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 enlargement as lies in 

 front of the end of the notochord, bends down at right angles 

 to the rest ; so that the anterior enlargement, or aoiterior cere' 

 bral 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). The under surface of the anterior vesicle lies in a 

 kind of pit, in front of, and rather below, the apex of the noto- 

 chord, 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 primi- 

 tively tubular spinal cord (Fig. 4, A). Where it does so, the 

 head ends, and the spinal column begins ; but no line of de- 

 marcation is at first visible between these two, the indifierent 

 tissues which ensheath the notochord passing Avithout inter- 

 ruption from one region to the other, and retaining the same 

 character throughout. 



The first essential differentiation between the skull and 

 the vertebral column is efi'ected by the appearance of the proto- 

 vertebroe. At regular intervals, commencing at the anterior 

 part of the cervical region, and gradually extending backward, 

 the indifi"erent 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 struct- 

 ure, and send arched prolongations into the walls of the spinal 

 canal, so as to constitute a proto vertebra. 



No protovertebrae appear in the floor of the skull, so that, 



