DEVELOPMENT OF THE EYE. 253 



In the embryo of jMan, as in that of all other Amphi- 

 rhina, two pear-shaped vesicles grow out laterally, at a very 

 early period, from the foremost part of the first brain - 

 bladder (Fig. 223, a, p. 218). These bladder-like protuberances 

 are the primary eye-vesicles. At first they are directed 

 outward and forward, but they soon make their way further 

 downward, so that after the specialization of the five brain- 

 bladders, they lie at the base of the twixt-brain. The 

 internal spaces within the two pear-shaped vesicles, which 

 soon attain a considerable size, communicate through their 

 hollow stalks with the cavity of the twixt-brain. Their 

 outer covering is formed by the outer skin-covering (horn- 

 plate and leather-plate). Where, on each side, the latter 

 comes directly in contact with the most curved portion of 

 the primary eye-vesicles, a thickening {I) arises, and at the 

 same time a groove-like indentation (0) in the horn-plate 

 (Fig. 242, 1). This groove, which we will call the lens groove, 

 changes into a closed sac, the thick- walled lens vesicle (2, Q, 

 owing to the fact that the. edges of the groove coalesce above 





y 



a- 



I- 



i 



IS 





w 



\ 



IV 



Fig. 242.— Eye of an embryonic Chick in longitudinal section (1, of a 

 germ after sixty-five hours of incubation ; 2, of a somewhat older germ ; 

 3, of a germ four days old) : ?i, hom-plate ; 0, lens groove ; Z, lens (in 1, 

 it still forms part of the epidermis, while in 2 and 3 it has separated); », 

 thickening of the hom-plate at the point from which the lens separated j 

 ;Z, vitreous body ; r, retina; u, pigment membrane. (After Remak.) 



