374 



THE PERIPHERAL NERVOUS SYSTEM 



groove of the fore-brain has not closed (Fig. 312). At 4 mm. the optic vesicles 

 are larger but still may be connected by a wide opening with the brain cavity 

 (Fig. 355 A, B). In the section shown in Fig. 355 C, the optic vesicle is at- 

 tached to the brain wall by a distinct optic stalk. 



The thickening, flattening and invagination of the distal and ventral wall of 

 the optic vesicle gives rise to the optic cup (Fig. 355 B, C, D). The area of in- 

 vagination extends ventrally along the optic stalk and produces the chorioid 

 fissure of the optic cup (Figs. 356 and 358). 



At the same time that the optic vesicle is converted into the optic cup, the 



Forebrain 

 Optic vesicle 



Attlage 

 of Lens 



Optic vesicle 



Relinal layer 

 D 



Fig. 355. — Stages in the early development of the eye. A, B, at 4 mm.; C, at 5 mm.; D, at 6.25 mm. 



(after Keibel and Elze). 



ectoderm overlying the former thickens, as seen in Fig. 355 B, forming the lens 

 plate, or optic placode. This plate invaginates to form the lens pit, the external 

 opening of which closes in embryos of 6 to 7 mm. (Fig. 355 D), producing the 

 lens vesicle which remains at first attached to the overlying ectoderm. In an 

 embryo of 10 mm. (Fig. 357) the lens vesicle has separated from the ectoderm, 

 which will form the epithelium of the cornea. The lens vesicle in earlier stages 

 is closely applied to the inner wall of the optic cup, but now it has separated from 

 it, leaving a space in which the vitreous body is developing. The inner retinal 

 layer of the optic cup has become very thick and is applied to its outer layer, so 



