DEVELOPMENT OF ELASMOBRANCH FISHES. 185 



of the optic vesicle towards the posterior takes place in quite 

 the normal manner; but, as has been already noticed by Gotte^ 

 and others, is not a simple mechanical result of the formation 

 of the lens, as is shewn by the fact that the vesicle assumes a 

 flattened form even before the appearance of the lens. The 

 whole exterior of the optic cup becomes invested by mesoblast, 

 but no mesoblastic cells grow in between the lens and the ad- 

 joining wall of the oj^tic cup. 



Round the exterior of the lens, and around the exterior and 

 interior of the optic cup, there appear membrane-like structures, 

 similar to those already described round the spinal cord and 

 other organs. These membrane-like structures appear with a 

 varying distinctness, but at the close of stage K stand out with 

 such remarkable clearness as to leave no doubt that they are 

 not artificial products (PL XIV. fig. 13a).^ They form the rudi- 

 ments of the hyaloid membrane and lens capsule. Similar, 

 though less well marked membranes, may often be seen lining 

 the central cavity of the lens and the space between the two 

 walls of the optic cup. The optic cup is at first very shallow, 

 but owing to the rapid growth of the free edge of its w^alls soon 

 becomes fairly deep. . The growth extends to the whole circum- 

 ference of the walls except the point of entrance of the optic 

 nerve (PL xiv. fig. 13a), where no grow^th takes place ; here 

 accordingly a gap is left in the walls, which forms the well 

 knowm choroid slit. While this double walled cup is increasing 

 in size, the wall lining the cavity of the cup becomes thick, and 

 the outer wall very thin (fig. 13a). No further differentiations 

 arise before the close of stage K. 



The lens is carried outwards with the growth of the optic 

 cup, leaving the cavity of the cup quite empty. It also 

 grows in size, and its central cavity becomes larger. Still later 

 its anterior wall becomes very thin, and its posterior wall 

 thick, and doubly convex (fig. loa). Its changes, however, 

 so exactly correspond to those already known in other Verte- 

 brates, that a detailed description of them would be superfluous. 



JS^o mesoblast passes into the optic cup round its edge, but a 

 process of mesoblast, accompanied by a blood-vessel, passes into 



1 Entii'ickelungRgeschlclite d. Unke. 



' The engraver has not been very successful in rendering these membranes. 



B. 13 



