198 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



In most mammals the sense of smell is well developed, but is comparatively 

 slight in the seals, whalebone whales and in the primates, while it is completely 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 destined may 



be recognized on its dorsal surface 

 before it is infolded to form the vesi- 

 cles of the brain. The accessory parts 

 of the eye are derived in part from the 

 general ectoderm, in part from meso- 

 derm of both kinds. 



As the neural plate closes up to form 

 the brain (p. n), 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 

 converted into a double walled optic 

 cup (fig. 204). 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. 204. Stereogram of de- 

 veloping eye. cf, chorioid fissure ;fb, 

 cut wall of fore-brain; /, anlage of 

 lens; oc, optic cup; os, optic stalk; p, 

 layer for pigmented epithelium; r, 

 retinal layer. 



-=-1 



FIG. 205. Sections of successive stages in the development of the lens of the eye from the 

 first thickening of the ectoderm (ec) to the complete separation ot 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 



