THE SENSE ORGANS. 131 



vesicle. At the same time a thickening of the deeper or ner- 

 vous layer of the surface epiblast takes place opposite the optic 

 vesicle : this grows rapidly and forms a spherical body, projecting 

 inwards from the surface : this is at first solid, but soon becomes 

 hollow and breaks away completely from the surface epiblast : 

 it becomes later on the lens of the eye, and may be spoken of 

 as the lens vesicle. 



Partly in consequence of the ingrowth of the lens vesicle, 

 and partly through growth of the optic vesicle itself, this latter 

 Ibecomes pitted on its outer surface, and so converted into a cup 

 — the optic cup — with double walls ; the inner wall being the 

 thickened and originally outer wall of the optic vesicle, and the 

 outer wall of the cup being the original inner or deeper part of 

 the wall of the vesicle. 



From the optic cup and lens vesicle the adult eye is derived 

 in the following way : The lens becomes solid, owing to thicken- 

 ing of its inner wall, which proceeds so far as to finally oblite- 

 rate the cavity. The optic cup enlarges considerably ; it remains 

 in contact with the lens at its edge or lip, but elsewhere is 

 separated from it by a space which becomes the posterior 

 chamber of the eye, and in which the vitreous humour is 

 iformed. The inner wall of the optic cup gives rise to the 

 retina, the rods and cones growing out from its outer surface ; 

 -while the outer and thinner wall of the optic cup forms the 

 layer of pigment cells in which the rods and cones are imbedded. 

 The choroid and sclerotic coats are formed from the mesoblast 

 surrounding the optic cup. 



The eye developes very slowly, and throughout the tadpole 

 stage of existence is in a very rudimentary and imperfect 

 condition. 



The Ear. The ears are developed as a pair of pit-like in- 

 vaginations of the nervous layer of the epiblast at the sides of 

 the hind-brain. The invaginations do not involve the epidermic 

 or surface layer of the epiblast, so that the auditory pits do not 

 • open to the exterior. 



The mouths of the pits very early narrow and close ; and the 

 auditory vesicles so formed separate from the epiblast and lie in 

 the mesoblast at the sides of the head. The vesicle becomes 

 the vestibule of the adult ear ; the semicircular canals arising 

 .as outgrowths from it. 



Throughout the tadpole stage of existence there is no further 



