THE NOSE AND EYE. 137 



the time that closure of the neural tube is effected ; they project 

 outwards at right angles to the axis of the head, their outer 

 walls being in close contact with the epidermis of the sides of 

 the head. Each of the vesicles becomes constricted at its base, so 

 as to form a spherical optic bulb, connected with the fore-brain 

 by a hollow tubular stalk. The outer wall of the bulb, which 

 is in contact with the external epidermis, soon becomes flattened, 

 and then thickens so greatly as almost to obliterate the cavity 

 of the vesicle (Fig. 67, oc). 



The lens. About this time a thickening of the inner, or 

 nervous, layer of the surface epiblast takes place opposite to the 



BF 



PT 



os 



TP 



OS 



FIG. 67. Transverse section through the head of a Tadpole of 6 mm. length, 

 about the time of hatching : the section passing through the fore-brain 

 and the developing eyes, x 45. 



AC, carotid artery. BF, fore-brain. DS, stomatodaeal imagination. NL, cuta- 

 neous or lateral line branch of the trigeminal nerve. OC, inner wall of optic cup. 

 OD, outer wall of optic cup. OL, lens. OS, optic stalk. PT, pituitary body. 

 TP, pharynx. VJ, jugular vein. 



centre of each optic vesicle ; this thickening increases rapidly, 

 and at the time of hatching of the tadpole forms a solid 

 spherical body projecting inwards from the surface ; this soon 

 becomes hollow, by breaking down of the cells in its centre, and 

 then separates from the surface epiblast. It may now be spoken 

 of as the lens vesicle (Fig. 66, OL) ; in the later stages, after 

 the formation of the mouth opening, the lens vesicle becomes 

 solid once more (Fig. 67, OL), mainly through lengthening of 

 the cells of its inner wall ; and by further increase in size it 

 becomes the lens of the adult eye. 



