THE VITREOUS HUMOUR. 41 I 



me. The lens capsule arises at about the same period as the 

 hyaloid membrane, and is a product 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 epi- 

 blastic lens, and not of the mesoblast, lies mainly iit the fact of 

 there being no mesoblast at hand to give rise to it at the time of 

 its formation, vide PL 15, fig. 13^. If the above observations 

 are correct, it is clear that the hyaloid membrane and lens 

 capsule are respectively products of the retina and lens ; so that 

 it becomes necessary to go back to the older views of Kb'lliker 

 and others in preference to the more modern ones of Lieberkiihn 

 and Arnold. It would take me too far from my subject to 

 discuss the arguments used by the later investigators to main- 

 tain their view that the hyaloid membrane and lens capsule are 

 mesoblastic products ; but it will suffice to say that the con- 

 tinuity of the hyaloid membrane over the pecten in birds is no 

 conclusive argument against its retinal origin, considering the 

 great amount of apparently independent growth which mem- 

 branes, when once formed, are capable of exhibiting. 



Bergmeister's and my own observations on the vitreous 

 humour clearly prove that this is derived from an ingrowth 

 through the choroid-slit. On the other hand, the researches 

 of Lieberkiihn and Arnold on the Mammalian Eye appear to 

 demonstrate that a layer of mesoblast becomes in Mammalia 

 involuted with the lens, and from this the vitreous humour 

 (including the membrana capsulo-pupillaris] is said to be in part 

 formed. Lieberkiihn states that in Birds the vitreous humour 

 is formed in a similar fashion. I cannot, however, accept his 

 results on this point. It appears, therefore, that, so far as is known, 

 all groups of Vertebrata, with the exception of Mammalia, con- 

 form to the Elasmobranch type. The differences between the 

 types of Mammalia and remaining Vertebrata are, however, not 

 so great as might at first sight appear. They are merely de- 

 pendent on slight differences in the manner in which the mesoblast 

 enters the optic cup. In the one case it grows in round one 

 specialized part of the edge of the cup, i.e. the choroid-slit ; in 

 the other, round the whole edge, including the choroid-slit. Per- 

 haps the mode of formation of the vitreous humour in Mammalia 

 may be correlated with the early closing of the choroid-slit. 



27 2 



