178 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



being pushed within the other. It is invaginated by the lens -bud 

 in the same manner as a schoolboy's fist indents a punctured 

 india-rubber ball. The invaginated vesicle is known as the 

 optic cup. The invagination of the vesicle, which takes place in 



. chor. fis. f 0n Monro 

 cerebr. vesicle / /_ 



3rd ventricle 



Olfact 

 lamina term 

 choroid, dep. 



optic recess 

 pituitary 



r right op. ves. 

 I turned down 



Fig. 145. — Diagram showing the condition of the Optic Stalk and Vesicle at the 

 commencement of the 2nd month. (After His.) 



an oblique manner — the pressure being applied from below and 

 behind, leads to the closure not only of the cavity of the vesicle, 

 but also to that of the distal half of the stalk (optic nerve). The 

 mesoblast, surrounding the lens, grows into the invagination and 

 afterwards forms the vitreous humour. The artery, which is 

 folded in with the mesoblast, becomes afterwards the central 

 artery of the retina. Hence the point at which the central artery 

 enters the optic nerve marks the upper limit of the invagination 

 of the optic stalk. By the fourth week the optic vesicle no longer 

 communicates with the cavity of the fore-brain but the recessus 

 opticus, in the floor of the third ventricle, above the chiasma, 

 marks the point at which it entered (Fig. 145). The optic 

 fibres, developed as processes of the neuroblasts of the invaginated 

 layer, grow into the brain from the retina along the optic stalk. 

 They thus form the greater number of the fibres in the optic 

 nerve. The optic fibres also form the chiasma in the floor of 

 the third ventricle and the optic tracts on the wall of the fore- 



