XOTE Oy THE EYES OF OARDTUM MUTICUM Reeve. 285 



cells. Tt enlarges by the division of the ceils and assumes an ellijî- 

 soidnl form (tig. 10). Cells forming the lower part of the ellipsoidal 

 mass become small by division. These smaller cells are not clearly 

 distinguished from larger cells, as there are many cells of infermediate 

 size. J^ater, liowever, the smaller cells are separated from the larger 

 ones and form the retinal portion, while the larger become the lens. 

 The later stages «^f dcveloj)ment are quite like those in the first process. 



In tlie two jyrocess of development, botli the lens and tlie retina 

 are produced from the epiblast. The process of their formation is 

 |)robably abbreviated. Originally thev were perha))s produced l)y two 

 separate invaginations as in the case of the Vertebrata, one invagina- 

 tion for the retina and one for the lens. Tlie invagination for tlu; 

 I'etina must ha\e l>een the first to Ix^ closed and cut off from the 

 epithelium ; it formed a hollow sphere, the upper wall of which 

 became the retina and the lower wall the choroid. The invagination 

 ior the lens was next formed and cut off in its turn from the epithe- 

 liuu). and had its lumen obliterated. 



The eyes of CardiiDii differ from those of Vecleir chiefly by the 

 presence of the choroid between thci retina and the tapetum and by the 

 mesoblastic origin of the pigment layer ; but I am inclined to think 

 that i n Pecten the choroid disappeared after secreting the tapetum, and 

 that the red pigment layer is mesoblastic in origin, and not directly 

 in connection with the retina. 



* It is hard far mo to accept Patten's ohsorvation that tlie lens of Pecten is mesoblastic in 

 oiigin. 



