EMBRYOLOGY OF TADPOLE AND CHICK 



579 



: L- THE EYE (Fig. 338) 



The general method of the eye formation is quite similar to that 

 already described in the chick. 



As the outer free rim of the optic cup draws tog-ether, it leaves a 

 small opening" which is the rudiment of the pupil. At this time we can 

 distinguish the inner and outer layer to the cup, and a central cavity. 

 These are the beginnings of the true retinal layer, the pigment layer, and 

 the posterior chamber of the eye respectively. 



There is a choroid fissure formed, just as in the chick. 



The lens forms as a thickening of the ectoderm opposite the pupil, 

 but this thickening involves only the nervous layer of the ectoderm. 

 It develops quite similarly to the ectodermal placodes in the formation 

 of the cranial nerves. In fact, the lens placode lies immediately anterior 

 to the placode of the V cranial nerve. About the time of hatching, the 

 lens has formed a prominent rounded thickening entirely cut <Mf from 

 the ectoderm. 



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jnes 



Fig. 338. 



Section through the eye of a tadpole at the time when the operculum is 

 forming, much magnified, ch, choroid ; cor, mesoblast cells which will give 

 rise to the cornea ; dm, Descemet's membrane ; ep, pigmented external 

 epithelium ; L, lens ; mes, branched mesoblast cells ; op.n., optic nerve ; 

 pch, pigmented epithelium of the choroid ; R, retina ; rd, rods and cones 

 pulled away from the pigmented epithelium of the choroid by contraction 

 of the preparation; vit, cells of the vitreous humour. (After Bourne.) 



This spheroidal mass hollows out, although it again becomes solid 

 by the cells on its inner side elongating, while the outer side remains as 

 a thin epithelial layer covering the distal surface of the lens. Then, as 

 the pupil narrows, the lens comes to lie just within the openings of the 

 cup. 



