176 Vertebrate Embryology 



As has already been described, the first in- 

 dication of the formation of the eye is seen on 

 the second day, when the optic vesicles are 

 pushed out from the sides of the fore-brain. 

 By the end of the second day these vesicles 

 are very prominent, and are constricted at their 

 bases to a narrow stalk, known as the optic 

 ' stalk (Fig. 60, O S). The optic stalk is hollow 

 and connects the optic vesicle with the lower 

 part of the fore-brain. At the end of the sec- 

 ond day, a slight thickening is seen in the 

 superficial ectoblast at the nearest point to 

 the outer wall of the optic vesicle (Fig. 59, 

 L). This is the first indication of the lens. 

 This thickening becomes pitted in to form, by 

 the fusion of the lips of the pit, a closed sac 

 (Figs. 59, L, and 6o r L), the lens vesicle. The 

 lens vesicle, during the third day, separates 

 from the superficial ectoblast, and the latter be- 

 comes again a continuous layer (Fig. 60, L). 

 The outer wall of the lens vesicle, after its 

 separation from the ectoblast, remains thin ; 

 while the inner wall becomes thicker and 

 thicker, by the growth and elongation of its 

 constituent cells, until, on the fourth day, it 

 comes in contact with the thin front wall and 

 entirely obliterates the cavity of the vesicle. 



