XI EAR 571 



diately surrounding the optic cup 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 

 vesicle, 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 Fig. 149). 



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 and 155, au.s) which deepens and becomes 

 flask-shaped ; and finally, as a rule, loses its connection with 

 the external ectoderm, becoming a closed sac surrounded by 

 mesoderm in which the cartilaginous auditory capsule is 

 subsequently developed. At first simple, it soon becomes 

 divided by a constriction into dorsal and ventral compart- 

 ments, from the former of which arise the utriculus and 

 semicircular canals, and from the latter the sacculus and 

 cochlea. 



The mode of development of the enteric canal has 

 already been dealt with (pp. 204 — 210). The first traces 

 of the liver and pancreas are seen as simple offshoots of the 

 mesenteron (archenteron), which gradually become branched 

 in a complicated manner, the numerous lobules being more 

 or less closely connected together by mesoderm. The 

 gill-pouches arise as paired outgrowths of the endoderm 

 lining the j pharynx which come into contact with the 



