140 THE THIRD DAY. [CHAP. 



Lastly, a section, taken perpendicular to the plane of 

 the paper along the line z, i.e. through the fissure 

 itself, would present the appearances of Fig. 50 C, 

 where the wall of the vesicle is entirely wanting in the 

 region of the fissure marked by the position of the 

 letter /. The external epiblast has been omitted in 

 the figure. 



The fissure such as we have described it exists for 

 a short time only. Its lips come into contact, and 

 unite (in the neighbourhood of the lens, directly, but in 

 the neighbourhood of the stalk, by the intervention of 

 a structure which we shall describe presently), and thus 

 the cup-like cavity of the secondary optic vesicle is 

 furnished with a complete wall all round. The interior 

 of the cavity is filled by the vitreous humour, a clear 

 fluid in which are a few scattered cells. 



With reference to the above description, two points require 

 to be noticed. Firstly it is extremely doubtful whether the 

 invagination of the secondary optic vesicle is to be viewed as an 

 actual mechanical result of the ingrowth of the lens. Secondly 

 it seems probable that the choroid fissure is not simply due to a 

 deficiency in the growth of part of the walls of the secondary 

 optic cup, but is partly due to a more complicated inequality of 

 growth resulting in a doubling up of the primary vesicle from 

 the side along the line of the fissure, at the same time that the 

 lens is being thrust in in front. In Mammalia, the doubling up 

 involves the optic stalk, which becomes flattened (whereby its 

 original cavity is obliterated) and then folded in on itself, so as 

 to embrace a new central cavity continuous with the cavity of 

 the vitreous humour. 



During the changes in the optic vesicle just de- 

 scribed, the surrounding mesoblast takes on the cha- 

 racters of a distinct investment, whereby the outline of 



