98 THE THIRD DAY. [CHAP. 



the rudiment of the lens. The small cavity within it speedily 

 becomes still less by the thickening of the walls, especially 

 of the hinder one. 



At its first appearance the lens is in immediate contact 

 with the anterior wall of the secondary optic vesicle (Fig. 

 27 .B). In a short time however, the lens is seen to lie in 

 the' mouth of the cup (Fig. SOD), a space (vh) (which is 

 subsequently occupied by the vitreous humour) making its 

 appearance between the lens arid 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. 



FIG. 28. 



DIAGRAMMATIC SECTION OF THE EYE AND THE OPTIC NERVE AT AN 

 EARLY STAGE (from Lieberkiihn), 



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 ol the cavity of the stalk s' with 

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



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 involu- 

 tion of the lens takes place, the direction 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. 27, but forms an obtuse angle with that 

 axis, after the manner of Fig. 28, where s represents the 

 cavity of the stalk leading away from the almost obliterated 

 cavity of the primary vesicle. 



Fig. 28 represents the early stage at which the lens fills 

 the whole cup of the secondary vesicle. The subsequent 

 state of affairs is brought about through the growth of the 



