4IO DEVELOPMENT OF ELASMOBRANCH FISHES. 



is there seen to be composed of mesoblast-cells with fibrous 

 prolongations. The cells, as has been noticed by Bergmeister, 

 form a special border round its dilated extremity. This pro- 

 cess is formed much earlier than the vitreous humour, which is 

 first seen in stage O. In hardened specimens this latter appears 

 either as a gelatinous mass with a meshwork of fibres or (as 

 shewn in PI. 17, 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 represent 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 (PI. 15, fig. 13^), 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 C> 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. 17, fig. 6, ky. m). 

 When the vitreous humour becomes artificially separated from 

 the retina, the hyaloid membrane sometimes remains attached 

 to the former, but at other times retains in preference its attach- 

 ment 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 de- 

 scribed 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 ap- 

 pears at a period subsequent to the latest one investigated by 



