VI.] 



THE EYE. 



133 



with the fore-brain are short and wide. The con- 

 strictions which give rise to the stalks take place chiefly 

 from above downwards, and also somewhat inwards and 

 backwards. Thus from the first the vesicles appear to 

 spring from the under part of the fore-brain. 



These stalks soon become comparatively narrow, 

 and constitute the rudiments of the optic nerves (Fig. 

 46 b). The constriction to which the stalk or optic 



Fig. 46. 





Section through the head of an embryo Teleostean, to 



SHEW THE formation OF THE OPTIC VESICLES, ETC. (From 



Gegenbaur ; after Sclienk.) 



c. fore-brain ; a. optic vesicle ; b. stalk of optic vesicle ; d. 



epidermis. 



nerve is due takes place obliquely downwards and 

 backwards, so that the optic nerves open into the base 

 of the front part of the thalamencephalon (Fig. 46 b). 



While these changes have been going on in the 

 optic stalks, development has also proceeded in the 

 region of the vesicles themselves, and given rise to the 

 rudiments of the retina, lens, vitreous humour, and 

 other parts of the eye. 



