EYES 



213 



In most mammals the sense of smell is well developed, but it is compara- 

 tively slight in the seals, whalebone whales and in the primates, while it is com- 

 pletely lost in most of the toothed whales where even the olfactory nerve may 

 disappear. 



THE EYES 



The sensory part of the eyes comes from the ectoderm of the 

 neural plate, and in several embryos the regions which are thus des- 

 tined may be recognized on its dorsal 

 surface before it is infolded to form the 

 vesicles of the brain (fig. 151, e). The 

 accessory parts of the eye are derived 

 in part from the general ectoderm, in 

 part from mesoderm of both kinds. 



As the neural plate closes up to form 

 the brain (p. 12), the optic areas begin 

 to grow outward from the fore-brain 

 toward the sides of the head, each form- 

 ing at first a hollow outgrowth, the optic 

 vesicle, connected with the brain by a 

 hollow optic stalk. The next phase is 

 the involution or invagination of the 

 distal side of the vesicle so that it is con- 

 verted into a double- walled optic cup 

 (fig. 228). There thus results a differentiation of parts in the optic 

 outgrowth and a partial obliteration of the cavity of the vesicle. 

 The distal wall, which forms the inside of the cup is called the retinal 



Fig. 228. — Stereogram of de- 

 veloping eye. cf, chorioid fissure; 

 }b, cutjwaU of fore-brain; /, aniage 

 of lens; oc, optic cup; cs, optic 

 stalk; />, layer for pigmented epi- 

 thelium; f, retinal layer. 



Fig. 229. — Sections of successive stages in the development of the lens of the eye from 

 the first thickening of the ectoderm (cc) to the complete separation of the lens, /. 



layer; the outer wall the pigment layer, in anticipation of their de- 

 velopment into the corresponding parts of the adult. 



The involution of the retina is not easily described, but may be 

 understood from fig. 228. It occurs on the lower distal side so that 



