DEVELOPMENT OF ELASMOBRANCH FISHES. 187 



or (as shewn in PI. xvi. fig. 6) with elongated fibres proceeding 

 from the end of the processus falciformis. These fibres are 

 probably a product of the hardening reagent, but perhaps re- 

 present some preformed structure in the vitreous humour. I 

 have failed to detect in it any cellular elements. It is more or 

 less firmly attached to the hyaloid membrane. 



On each side of the processus falciformis in stage P a slight 

 fold of the optic cup is to be seen, but folds so large as those 

 represented by Bergmeister have never come under my notice, 

 though this may be due to my not having cut sections of such 

 late embryos as he has. The hyaloid membrane appears long 

 before the vitreous humour as a delicate basement membrane 

 round the inner surface of the optic cup (PL XIV. fig. 18a), which 

 is perfectly continuous with a similar membrane round the outer 

 surface. In the course of development the hyaloid membrane 

 becomes thicker than the membrane outside the optic cup, with 

 which however it remains continuous. This is very clear in my 

 sections of stage M. By stage the membrane outside the cup 

 has ceased to be distinguishable, but the hyaloid membrane 

 may nevertheless be traced to the very edge of the cup round 

 the developing iris; but does not unite with the lens capsule. 

 It can also be traced quite to the junction of the two 

 layers of the optic cup at the side of the choroid slit 

 (PI. XVI. fig. 6, hy. m). When the vitreous humour becomes 

 artificially separated from the retina, the hyaloid meml)rane 

 sometimes remains attached to the former, but at other 

 times retains in preference its attachment to the retina. My 

 observations do not throw any light upon the junction of the 

 hyaloid membrane and lens capsule to form the suspensory 

 ligament, nor have I ever seen (as described by Bergmeister) 

 the hyaloid membrane extending across the free end of the 

 processus falciformis and separating the latter from the vitreous 

 humour. This however probably appears at a period subsequent 

 to the latest one investigated by me. The lens capsule arises at 

 about the same period as the hyaloid membrane, and is a pro- 

 duct of the cells of the lens. It can be very distinctly seen in 

 all the stages subsequent to its first formation. The proof of 

 its being a product of the epiblastic lens, and not of the 

 mesoblast, lies mainly in the fact of there being no mesoblast 



13—2 



