THE ORGANS OF SPECIAL SENSE. 



575 



The Lens. — As each optic vesicle grows out toward the surface, its outer 

 wall soon comes to lie just beneath the surface ectoderm. The cells of that 

 portion of the ectoderm which overlies the optic vesicle next proliferate and 

 cause a thickening of the ectoderm (Fig. 498, left side). This thickening of the 



Pore-brain vesicle 



Lens area 



Optic vesicle 



Surface ectoderm 



Optic vesicle 



Fig. 498. — Section through head of chick of two days' incubation. Duval. 

 The formation of the optic vesicle and stalk appears to be somewhat more advanced 

 - on the left than on the right. 



ectoderm over the optic vesicle is apparent in the chick embryo of 36 hours in- 

 cubation; in the human embryo it occurs about the third or fourth week and 

 represents the first step in the development of the crystalline lens. The thick- 

 ened portion of ectoderm is known as the lens area (Fig. 498). The latter next 



Fore-brain 



Lens invagination 



Optic vesicle 



- - Lens invagination 



Optic vesicle 

 Fig. 499.— Section through head of chick of three days' incubation. Duval. 



becomes depressed against the outer surface of the optic vesicle forming a 

 distinct lens invagination (Fig. 499). This becomes cup-shaped and then its 

 edges come together and fuse, thus forming the lens vesicle (Fig. 500) . At first the 

 lens vesicle is connected with the surface ectoderm, but about the eighth week 

 37 



