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. 11), the optic areas begin 
~_, to grow outward from the fore-brain 
Mg toward the sides of the head, each form- 
ieee ing at first a hollow outgrowth, the optic 
vesicle, connected with the brain by a 
Fic. 204.—Stereogram of de- 
veloping eye. cf, chorioid fissure; fb, hollow optic stalk. The next phase is 
cut wall of fore-brain; /, anlage o ? . F bs e 
lens; oc, optic cup; os, optic stalk; , the involution or invagination of the 
layer for pigmented epithelium; 7, 
retinal layer. 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 
ES 
Ay 
Bec ooHuue! 
Fic. 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 
