THE PROCESSUS FALCIFORMIS. 409 



wall becomes very thin, and its posterior wall thick, and doubly 

 convex (fig. 13^). Its changes, however, so exactly correspond 

 to those already known in other Vertebrates, that a detailed 

 description of them would be superfluous. 



No mesoblast passes into the optic cup round its edge, but a 

 process of mesoblast, accompanied by a blood-vessel, passes into 

 the space between the lens and the wall of the optic cup through 

 the choroid slit (fig. 1 3^, c/i). This process of tissue is very easily 

 seen, and swells out on entering the optic cup into a mushroom- 

 like expansion. It forms the processus falciformis, and from it 

 is derived the vitreous humour. 



About the development of the parts of the eye, subsequently 

 to stage K, I shall not say much. The iris appears during 

 stage O, as an ingrowing fold of both layers of the optic cup 

 with a layer of mesoblast on its outer surface, which tends to 

 close over the front of the lens. Both the epiblast layers com- 

 prising the iris are somewhat atrophied, and the outer one is 

 strongly pigmented. At stage O the mesoblast first also grows 

 in between the external skin and the lens to form the rudiment 

 of the mesoblastic structures of the eye in front of the lens. The 

 layer, when first formed, is of a great tenuity. 



The points in my observations, to which I attach the 

 greatest importance, are the formation of the lens capsule and 

 the hyaloid membrane ; with the development of these may be 

 treated also that of the vitreous humour and rudimentary pro- 

 cessus falciformis. The development of these parts in Elasmo- 

 branchs has recently been dealt with by Dr Bergmeister 1 , and 

 his observations with reference to the vitreous humour and 

 processus falciformis, the discovery of which in embryo Elas- 

 mobranchs is due to him, are very complete. I cannot, however, 

 accept his view that the hyaloid membrane is a mesoblastic pro- 

 duct. Through the choroid slit there grows, as has been said, 

 a process of mesoblast, the processus falciformis, which on 

 entering the optic cup dilates, and therefore appears mushroom- 

 shaped in section. At the earliest stage (K) a blood-vessel 

 appeared in connection with it, but no vascular structure came 

 under my notice in the later stages. The structure of this 

 process during stage P is shewn in PI. 17, fig. 6, /. fal. ; it 



1 " Embryologie d. Coloboms," Sitz. d. k. Akad. Wien, Bd. LXXI. 1875. 

 B. 27 



