CHAP, xm EYE AXD EAR 595 



is differentiated to form the choroid, the iris, and the 

 sclerotic. 



Thus the eye of Vertebrates has a threefold origin : the 

 sclerotic, choroid, iris, vitreous humour, and the greater 

 part of the cornea are mesodermal ; the lens and external 

 epithelium of the cornea are derived from the ectoderm of 

 the head : the retina and optic nerve are developed from a 

 hollow pouch of the brain, and are therefore in their ultimate 

 origin ectodermal. The sensory cells of the retina the 

 rods and cones although not directly formed from the 

 external ectoderm, as in Invertebrates, are ultimately 

 traceable into the superficial layer of ectoderm, since they 

 are developed from the inner layer of the optic cup, which 

 is a prolongation of the inner layer of the brain, the latter 

 being continuous, before the closure of the medullary groove, 

 with the ectoderm covering the general surface of the body 

 (compare Figs. 155 and 157). 



The organ of hearing, like that of smell, arises in the 

 embryo as a paired invagination of the ectoderm in the 

 region of the hind-brain, a shallow depression being 

 formed (Figs. 64, L, 157, A. and 158, an. s) which deepens 

 and becomes flask-shaped ; and finally, as a rule (com- 

 pare p. 465), loses its connection with the external 

 ectoderm, giving rise to a closed sac surrounded by 

 mesoderm in which the cartilaginous auditory capsiile 

 is subsequently developed. At first simple, it soon 

 becomes divided by a constriction into dorsal and 

 ventral compartments, from the former of which arise 

 the utriculus and semicircular canals, and from the 

 latter the sacculus and cochlea. 



The early development of the enteric canal has already 

 been dealt with in the case of the frog (pp. 204 and 

 210), and it will be remembered that the greater part of it 

 (mesenterori) is lined by endoderm, its cavity being at 

 first bounded by the yolk below, but gradually becom- 

 ing closed in by endoderm cells. \Vhen a yolk-sac is 

 formed (dogfish, bird, mammal, Figs. 129, 159, 166), it 



