188 THE HYALOID MEMBRANE. 



at hand to give rise to it at the time of its formation, 

 vide PI. XIV, fig. 13 a. If the above observations are cor- 

 rect, 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 Kol- 

 liker 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 in- 

 vestigators to maintain their view that the hyaloid membrane 

 and lens capsule are mesoblastic products; but it will suffice 

 to say that the continuity of the hyaloid membrane over the 

 pec ten in birds is no conclusive argument against its retinal 

 origin, considering the great amount of apparently independent 

 growth which membranes, 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 memhrana caj^sulo-pupillaiHs) 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 excep- 

 tion of Mammalia, conform 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 dependent 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. Perhaps the mode of forma- 

 tion of the vitreous humour in Mammalia may be correlated 

 with the early closing of the choroid-slit. 



Auditory Organ. With reference to the development of the 

 organ of hearing I have very little to say. Opposite the inter- 

 val between the seventh and the glosso-pharyngeal nerves the 



