132 



THE THIRD DAY. 

 FIG. 45. 



[CHAP. 



cfa 



SECTION THROUGH THE DORSAL REGION OF AN EMBRYO DOG-FISH. 



pr. posterior root ; sp.g. spinal ganglion ; n. nerve ; x. attach- 

 ment of ganglion to spinal cord ; nc. neural canal ; mp. 

 muscle-plate ; ck. notochord ; i. investment of spinal cord. 



which subsequently join the posterior roots below the 

 ganglia. The shape of the root of a completely formed 

 spinal nerve, as it appears in an embryo of the fourth 

 day, is represented in Fig. 68. 



The Eye. In the preceding chapter we saw how 

 the first cerebral vesicle, by means of lateral outgrowths 

 followed by constrictions, gave rise to the optic vesicles. 

 These and the parts surrounding them undergo on the 

 third day changes which result in the formation of the 

 eyeball. 



At their first appearance the optic vesicles stand 

 out at nearly right angles to the long axis of the 

 embryo (Fig. 27), and the stalks which connect them 



