DEVELOPMENT OF THE SENSE ORGANS 



377 



embryo of 10 mm. (Fig. 376) the lens vesicle has separated from the ectoderm, 

 which will form the epithehum of the cornea. The lens vesicle in earher stages 



A B 



Anlage of lens 



Fore-brain 



Optic vesicle 

 Optic stalk 



Optic vesicle 



Optic vesicle 



D 



Retinal layer 



Lens vesicle 



Dienccplmlon—^, 



Fig. 374. — Stages in the early development of the human eye. A, 5, at 4 mm. (X 27); C, at 5 mm. (X 

 23); D, at 6.25 mm. (X 18) (after Keibel and Elze). 



(Fig. 374 D) 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 apphed to the outer layer, 

 so that the cavity of the primi- 

 tive optic vesicle is nearly ob- 

 literated (Fig. 376). Pigment 

 granules have begun to appear in 

 the outer cells which form the 

 pigment layer of the retina. Mes- 

 enchymal tissue surrounds the 

 optic cup and is beginning to 

 make its way between the lens 

 vesicle and the ectoderm. Here 



the anterior chamber of the eye develops later as a cleft in the mesoderm. The 

 distal mesenchymal tissue (next the ectoderm) forms the substantia propria of the 



Ci \ stalline lens 



Chorioid fissure 



Optic stalk 



Fig. 375. — The optic stalk, cup and lens of a human 

 embryo of 12.5 mm. The chorioid fissure has not yet ex- 

 tended along the optic stalk (from Fuchs, after Hoch- 

 stetter). X 90. 



