TYPES OF VERTEBRATE AND INVERTEBRATE EYES 15 



sc. 



,.c popv. 



Fig. 10. 



A — Section through the optic vesicle of a human embryo, 7 mm., showing con- 

 tinuity of the cavity of the primary optic vesicle with the cavity of the third 

 ventricle. The surface (SC.) of the invaginated layer of the optic cup was 

 originally on the superficial aspect of the medullary plate. L. : lens vesicle. 



B — Schematic diagram of a vertical section of the optic cup of the 7-mm. human 

 embryo shown in A, representing the positions of the retinal layers when these 

 are fully differential, in order to demonstrate the relation of the rod-and-cone 

 layer of sensory cells to the cavity of the primary optic vesicle, the lining 

 epithelium of which was originally on the superficial surface of the neural- 

 plate, and later formed the epithelium at the bottom of the optic groove, 

 before the closure of the medullary folds. The nerve-fibre layer of the retina 

 corresponds to the deep surface of the neural plate. When the optic cup is 

 formed it becomes directed towards the outer surface of the cup by the 

 inversion of the ventro-lateral segment of the primary optic vesicle. The 

 arrows indicate the direction of the decussating and tract fibres of the optic 

 nerve in the region of the future chiasma. 



hi. p. : bipolar cells. 



c.p.op.v. : cavity of primary optic 



vesicle. 

 g.c. : ganglion cells. 

 /. ; lens. 



nf.l. : nerve-fibre layer. 



p.l. : pigment layer of retina. 



s.ep. : sensory layer (rods 



cones). 

 III. V. : third ventricle. 



and 



