570 



EVE 



CHAP. 



small ca\ity and lliick walls (compare Fig. 64, I, c). Thi.s 

 liody is tlic rudiment of ihc Kns : as it enlarges it |)ushes 

 against tlic ()|itic \'esi(ie and causes it tci become in- 

 \'aginatcd (/>'), tlie singlcda)'cred optic \esicle thu,s he- 

 cnmes <i)n\irled into a Iwodascred uplic tup {opt. f, opt.c), 

 its cavil)-, originally continuous with the ttiac(ele, becoming 

 obliterated, lietween the edge oT the cup and the lens, on 

 the ventral side, is a small space which graduallv extends 

 towards ihi; stalk of the cup, and thus gi\"es rise to a slit 



in the wall of the latter : this 



choroid fiisurc (I'ig. 1 so, aiis), 

 as it is called, soon becomes 

 closed 1))' the union of its 

 edges. The outer la_\'er of 

 the optic cup becomes the 

 pigmentdayer of the retina 

 (p. 183) : from its inner layt'r 

 the rest of that membrane — 

 including the rods and cones 

 — is formed. The stalk of the 

 optic cup occupies, in the 

 embryonic eve, the place of 

 the optic nerve, but the actual 

 fibres of the nerve are formed from the ner\'e-cells of ihi' 

 retina and grow inwards to the brain. 



During the formation of the lens, mesoderm extends in 

 between the ingrowth from which it arises and the external 

 c'Ctoderm ; from this the main substance of the cornea and 

 its inner or posterior epithelium are formed, the adjacent 

 ectoderm becoming the external epithelium, i.e. that of 

 the conjiuii tiva (p. 182). Mesoderm also makes its way 

 into the optic cup, through the choroid fissure, and gives 

 risi- to the vitre(jus humour, Lastlv, the mesoderm inimc- 



'I'l.^T^A. — Pl.'\',(ir rejire^enl.'ill.i 

 optii: cii|> aiul len^. 

 ((/'.I fjutt/r wall of optic cup : aii^. 

 clinroid Hs^ure ; .<;L cavity of optic 

 r Lip ; //. space between the two 

 w.'ill-,, which afterward.'? disappears ; 

 //'. inner wall of optic cup ; /. lens ; 

 .S>i. stalk r,f optic cup (rudiment of 

 i.plic nerxe). (.Vfler Herl«ia.) 



