234 BULLETIN: MUSEUM OF COMrAKATIVE ZOOLOGY. 



membrane (Htille). Ten years later Babucliin ('65) made the promising 

 discovery that the peripheral layer of the cylindrical rod of Limax is 

 radially striated, and Hensen ('66, pp. 42-45) proved that the axial part 

 of the rod in Pteroceras contains longitudinal fibrils. Babuchin stated 

 clearly, for the first time, that the retina is made up of definite cells 

 and that they have such a differentiation that the appearance of zones is 

 produced ; that some of the cells are sensory while others are doubtfully 

 so; and that the optic nerve has an immediate," not a ganglionic, con- 

 nection with the sensory cells. He showed that certain cells (which he 

 called " central cells") in Helix have cilia-like structures which make up 

 the central zone of the retina ; but that in Limax the same zone consists 

 of cylindrical structures with radially striated borders. He was thus 

 the first to picture neurofibrillae, although he did not understand their 

 nature. It is true that he was in error as to the character of the " cen- 

 tral cells" and the striated nature of the pigment cells; yet, if he had 

 interpreted the " central cell " with its " capital " as sensory and the 

 surrounding pigment cells as indifferent, he would have been in advance 

 of his time by a generation. See Fig. A. 



The observations of Babuchin and Hensen in regard to the fibrillar 

 nature of the rods, not having been interpreted physiologically, were 

 neglected. The dominating influence of Schultze's ('69) interpreta- 

 tion of the rod in cephalopods and heteropods contributed to this result. 

 Hence, when Hilger ('84) described the mantle of the rod of Helix as 

 cuticular, he merely accepted the prevalent belief that the rod is a sub- 

 stance secreted by the retinal cells. He found the same idea expressed 

 by Greff ('75, '76), who worked on the eyes of the Alciopidae, and by 

 Grenacher ('79), who studied the retina of arthropods. Hilger (com- 

 pare Fig. B.) characterized the mantle of the rods as completely homo- 

 geneous and structureless, as cuticular substance secreted jointly by 

 the pigment cells. He thus ascribed to the pigment cells in Helix struc- 

 tural conditions which they do not possess, and although the distal ex- 

 tension of the rod-cell occupies the centre of the rod in all the species he 

 studied, he saw no sign of fibrillae in it. 



Simroth ('76) entirely overlooked the rod in the eyes of Helix, Limax, 

 and Pteroceras; and Carriere ('85) omitted the rods in all the gastero- 

 pods, even in copying the figures of Fraisse ('8l), who also, like most 

 investigators of that period, looked upon the rod-zone as cuticular. 

 Btitschli ('84) agreed with Hilger in all essential particulars and applied 

 the same criteria to the retinas of other groups of animals. 



Patten ('86) protested that the light-recipient surface in arthropods 



