592 NERVOUS SYSTEM CHAP. 



fore-brain being strongly inclined to that of the hind-brain. 

 The bend is known as the cerebral flexure (Figs. 158 and 159) : 

 it is really permanent, but as the hemispheres grow forwards 

 parallel to the hind-brain, and the floor of the mid-brain and 

 hind-brain thickens, it becomes obscure and is not noticeable 

 in the adult. 



The central cavity of the embryonic spinal cord has 

 at first the form of a narrow vertical slit (Fig. 155, c), 

 the walls of which eventually fuse dorsally, while 

 ventrally part of the slit forms the central canal of the 

 adult spinal cord, in which dorsal and ventral fissures 

 are developed. 



The ganglia of the dorsal roots of the spinal nerves 

 are developed from a paired neural crest (Figs. 65 B, 

 161, and 164) arising close to the junction of the 

 medullary plate and outer ectoderm : this becomes seg- 

 mented to form the ganglia, from the cells of which the 

 sensory fibres arise and extend into the spinal cord. 

 The ventral roots arise as direct outgrowths from 

 cells in the ventral region of the medullary cord. 

 Certain of the cerebral nerves are developed in an 

 essentially similar manner to the dorsal roots of the 

 spinal nerves, while others *Qrise as direct ventral 

 outgrowths from the brain, like the ventral roots. The 

 fibres of all the typical nerves grow peripherally until 

 they reach the parts which they ultimately supply. 



The olfactory organs arise as sac-like invaginations of 

 the ectoderm, one on either side of the snout (Fig. 

 158), and become enclosed by the cartilaginous 

 olfactory capsules, developed, with the rest of the 

 skeleton, from the mesoderm. The aperture of invagina- 

 tion gives rise to the external nostril, the internal 

 nostrils (in air-breathing forms) being developed subse- 

 quently. 



The mode of development of the paired eye of verte- 

 brates is peculiar and characteristic. 



