THE ORGANS OF THE OUTER GERM-LAYER. 467 



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



of Smell. 



As the outer germ-layer is the parental tissue of the central 

 nervous system, so also does it form the substratuni for the higher 

 sensory organs, the eye, .the ear, and the organ of smell. For it 

 furnishes the sensory epithelium, a component which, in comparison 

 with the remaining parts, derived from the mesenchyma, is, it is 

 true, of very small volume, but, notwithstanding, by far the most 

 important both from a functional and a morphological point of 

 view. Whether a sensory organ is adapted for seeing, hearing, 

 smelling, or tasting depends primarily upon the character of its 

 sensory epithelium, i.e., upon whether it is composed of optic, 

 auditory, olfactory, or gustatory cells. But also morphologically 

 the epithelial part is preeminent, because it is chiefly this which 

 determines the fundamental form of the sensory organs and affords 

 the fixed centre around which the remaining accessory components 

 are arranged. The genetic connection with the outer germ-layer 

 may be most clearly recognised in many Invertebrates, inasmuch as 

 here the sensory organs are permanently located in the epidermis, 

 whereas in Vertebrates, as is well known, they are, for the sake of 

 -protection, embedded in deep-lying tissues. I begin with the eye, and 

 then proceed to the organ of hearing and that of smell. 



A. The Development of the Eye. 



As has already been stated in the description of the brain, the 

 lateral walls of the primary fore-brain (figs. 234, 263) are evaginated 



kh rf gb nil 



Fig. 263. Brain of a human embryo of the third week (Lg). Profile reconstruction, after His. 



yh, Cerebral vesicle ; zh, between-brain vesicle ; mh, mid-brain vesicle ; kh, nh, vesicles of cere- 

 bellum and medulla oblongata ; au, optic vesicle ; gb, auditory vesicle ; tr, infundibulum ; 

 rf, area rhomboidalis ; nb, nuchal flexure ; kb, cephalic flexure. 



and produce the primary optic vesicles (au), which are constricted 

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



