266 bulletin: museum of comparative zoology. 



genetic position of those genera in the group. The epithelial nature of 

 the retiua is again illustrated in the regeneration of the eye, which was 

 studied by Carriere ('80). That the indifferent (pigment) cells of the 

 retina of Limax (to which Hesse has ascribed the function of aiding the 

 corneal cells in the work of secreting the lens and vitreous humor) can 

 have any immediate relationship to the unicellular glands of the external 

 epithelium, as Carriere ('85) supposed, has not been substantiated by 

 evidence. Apparently Carriere's ideas were the natural outcome of his 

 earlier studies on regeneration. He traced the developmental history 

 of the eyes and observed the evident continuity of the retina with the 

 surface epithelium in molluscs having cup-shaped eyes. It therefore 

 seemed logical to homologize the retina directly with the surface epi- 

 thelium, the unicellular gland cells of the latter corresponding, in his 

 opinion, to the unpigmented cells of the retina, the tactile and other 

 cells, to the pigment cells of the retina. Such a line of reasoning 

 brought him to a conception of the function of the two kinds of retinal 

 cells, which, as the connections of the retinal cells with the optic nerve 

 and the use of methylen blue show, is incorrect. To decide whether 

 the pigment cells really do aid in secreting the lens and vitreous humor 

 will require further investigation. 



Emphasis should be placed on the way in which the retinal epithelium 

 is attached to the eye-capsule, which stands in the same relation to the 

 retina as an ordinary basement membrane to its overlying epithelium. 

 The proximal branching processes of the retinal cells, both pigmented 

 and sensory, in Limax and other gasteropods have been described re- 

 peatedly and variously interpreted. Babuchin supposed the branches of 

 the pigment cells to be nerve fibres. Being in doubt as to the function 

 of the " central cells " he was silent as to the function of their processes. 

 Hensen regarded the proximal part of the cells as a "fussformige Ab- 

 schnitt," while nearer the nucleus a supposed neurite passed off. Sim- 

 roth was the first to point out that the retinal cells were attached to the 

 capsule, but his observation seems to have been overlooked, possibly 

 because his work contains so much that is erroneous. He described his 

 third type of cell as ending in two or more root-like branches which 

 appeared in the capsule. But similar branches on the sensory cells lead 

 him to suppose some indirect connection with the optic nerve by means 

 of ganglion cells. Hilger, believing that all of the cells of the retina 

 were sensory, supposed that the proximal processes were connected with 

 the optic nerve either directly or through ganglion cells. Grenadier 

 ('86, p. 23) described the special organs of attachment of the sensory 



