136 THE THIRD DAT. [CHAP. 



is seen to lie in the mouth of the cup (Fig. 50 A), a 

 space (vh) (which is occupied by the vitreous humour) 

 making its appearance between the lens and anterior 

 wall of the vesicle. 



In order to understand how this space is developed, 

 the position of the optic vesicle and the relations of 

 its stalk must be borne in mind. 



The vesicle lies at the side of the head, and its 

 stalk is directed downwards, inwards and backwards. 

 The stalk in fact slants away from the vesicle. Hence 

 when the involution of the lens takes place, the direc- 

 tion in which the front wall of the vesicle is pushed in 

 is not in a line with the axis of the stalk, as for 

 simplicity's sake has been represented in the diagram 

 Fig. 47, but forms an obtuse angle with that axis, after 

 the manner of Fig. 48, where s represents the cavity 



Fig. 48. 



DlAORAMMATIC SECTION OF THE EXB AND THE OPTIC NeRVE 



AT AN EARLY STAGE (from Lieberktllm), 



to shew the lens I occupying the whole hollow of the optic cup, 

 the inclination of the stalk s to the optic cup, and the 

 continuity of the cavity of the stalk s' with that of the 

 primary vesicle c ; r, anterior, u posterior wall of the optic 

 cup. 



