ORGANS OF VISION OF THE VERTEBRATA. 505 



the mesoblast cells involuted with the lens. The anterior part of the 

 vascular sheath is however inclosed in a very delicate membrane, the 

 membrana pupillaris, continuous at the sides with the epithelium of 

 Descemet's membrane. On the formation of the iris this membrane lies 

 superficially to it, and forms a kind of continuation of the mesoblast of the 

 iris over the front of the lens. 



The origin of this membrane is much disputed. By Kessler, whose 

 statements have been in the main followed, it is believed to appear 

 comparatively late as an ingrowth of the stroma of the iris ; while Kolliker 

 believes it to be derived from a mesoblastic ingrowth between the front wall 

 of the lens and the epiblast. According to Kolliker this ingrowth subse- 

 quently becomes split into two lamina;, one of which forms the cornea, and 

 the other the anterior part of the vascular sheath of the lens with its mem- 

 brana pupillaris. Between the two appears the aqueous humour. 



The membrana capsulo-pupillaris is simply a provisional embryonic 

 structure, subserving the nutrition of the lens. The time of its disappear- 

 ance varies somewhat for the different Mammalia in which this point has 

 been investigated. In the human embryo it lasts from the second to the 

 seventh month and sometimes longer. As a rule it is completely absorbed 

 at the time of birth. The absorption of the anterior part commences in the 

 centre and proceeds outwards. 



In addition to the vessels of the vascular capsule round the lens, there 

 arise from the arteria centralis retinas, just after its exit from the optic nerve, 

 in many forms (Dog, Cat, Calf, Sheep, Rabbit, Man) provisional vascular 

 branches which extend themselves in the posterior part of the vitreous 

 humour. Near the ciliary end of the vitreous humour they anastomose with 

 the vessels of the membrana capsulo-pupillaris. 



In Mammals the choroid slit closes very early, and is not perforated 

 by any structure homologous with the pecten. The only part of the slit 

 which remains open is that perforated by the optic nerve ; and in the centre 

 of the latter is situated the arteria centralis retina; as explained above. 

 From this artery there grow out the vessels to supply the retina, which 

 have however nothing to do with the provisional vessels of the vitreous 

 humour just described (Kessler). On the atrophy of the provisional 

 vessels the whole of the blood of the arteria centralis passes into the 

 retina. 



It is interesting to notice (Kessler, No. 372, p. 78) that there seems to be 

 a blood-vessel supplying the vitreous humour in the embryos of nearly all 

 vertebrate types, which is homologous throughout the Vertebrata. This 

 vessel often exhibits a persisting and a provisional part. The latter in 

 Mammalia is the membrana capsulo-pupillaris and other vessels of the 

 vitreous humour ; in Birds and Lizards it is the part of the original vascular 

 loop, not included in the pecten, and in Osseous Fishes that part (?) 

 not involved in the processus falciformis. The permanent part is formed by 

 the retinal vessels of Mammalia, by the vessels of the pecten in Birds and 

 Lizards, and by those of the processus falciformis in Fishes. 



