THE ORGANS OF Tl | GERM-LA ^ 



II. The Development of the Sensory Organs, Eye, Ear, and Organ 



of Smell. 

 As the outer germ-layer is the parental tis>u.- of the o- 



ystem, so also does it form the sub lie higher 



>ry organs, the eyo, the ear, and the organ of smell. 1 

 inrnishes the sensory epithelium, a component whirh, in comparison 

 with tli. remaining parts, D the me- 



true, of \er\ Mnall volume, but. notwithstanding. most 



important Kith tYom nal and a morphological 



view. \Vli- ther a ensory organ is adapted for seeing, hearing, 

 smelling, or tasting depends primarily up. in the rhara. -NT 

 '-., ujx)n whether mposed of < 



auditory, olfai toi y. or irustatory cells. But also morphologically 

 tin- fpiilirlial p.u-t is preeminent, because it is chii-tly ti-.is wlii.-li 



the li.M-.l cTiittv aroiiml \\hidi tlic remaining accessory components 

 are arrani:'<l. The *:< Mnection \\-itli layer 



may ! most clearly recognisetl in n rates, inasmuch as 



In ! the sensory nriMns are permanently located in the epidermis, 

 \vIuTi aa in \'. rtel>rates, as is well known, they are, for the sake of 

 protection, embedded in deep lyini: tissues. I begin \\ ith th. 

 then proceed to the orga ring and ' :-ll. 



A. The Devel' the Eye. 



As has already In en Mated in the d, ..f the brain, the 



lateral walls of the primary fore-l>: :iin (figs. 234, 263) a: ated 



th */ yb nk 



Fig. 2G3.-Brain of a human embryo of the third WMk (Lg). Profile recoiutructioi. 



n vesicle; w*. mid-btmin vesicle; M, lO, Tdde* of o*w- 



iK-lluin ami ii.i-.luHa oblongata ; an, optic vesicle; 06, auditory vesicle ; Cr, infondibulum ; 

 rj\ ar, >\\ flexure ; *6, oepbaUo flexure. 



And produce the primary opnV V(\>icles (au), which aro constricted 

 off more and more and remain in connection with the between-brain 



