236 bulletin: museum of comparative zoology. 



the light-recipient organs in any group of animals are purely cuticular 

 structures has disappeared and the facts on which such theories were 

 based have been entirely reinterpreted. 



Shreiner ('97, p. 45) may be mentioned as one, who, as late as 1897, 

 missed the rods in the eye of Patella. 



Henchman ('97), taking up the structure of the sensory cell where 

 Babuchin left it in 1865, made a preliminary report on the eye of Limax 

 maxim us, in which she described the histological elements briefly. The 

 unpigmented cells were interpreted as sensory, the pigment cells as in- 

 different. Thus the correct relations were given for the first time. The 

 axis of the rod as well as the body of the sensory cells was described as 

 " longitudinally fibrous." The h'brillae of the mantle were also recog- 

 nized. No physiological interpretation of the fibrils was announced. 

 The sensory cells were found to pass into fibres proximally, where, it 

 appears, they were erroneously supposed to connect with the optic nerve 

 indirectly, for an " optic ganglion " is described as located in the base of 

 the nerve. The accessory retina was described for the first time. 



Since then Hesse has published a long series of researches on the fibril- 

 lae of the rods in many groups of animals and has reinterpreted some 

 of the facts established by former investigators, bringing to his work a 

 painstaking technique and a unity of interpretation which have made 

 his researches notable. His sixth contribution (Hesse, :00), to which 

 reference will be made later, did not deal with the gasteropods. In 11)02 

 (Hesse, :02 a ) he gave a general summary of all his earlier work, and 

 likewise added new researches, which include a study of the retina of 

 gasteropods. Hesse has not used special methods for the differentiation 

 of neurofibrils, and has, therefore, seldom identified them in the cell 

 body ; hence, until now, our knowledge of them has been confined mainly 

 to the rod. With the classical work of Apathy ('97) to point the way, 

 he has shown the significance of the fibrillae of the rods of gasteropods 

 with a clearness which no one before him had reached. He interprets 

 the striated border of the rods, which Babuchin discovered in Helix 

 and Limax, as the recipient, fibrillar endings of the optic apparatus. He 

 believes that, with few exceptions, the essential feature of all light- 

 recipient organs is the presence of neurofibrillae. The axial fibrillae of 

 the rod, such as Hensen found in Pteroceras, he looks upon as intracel- 

 lular fibrils, which must find their way into the neurite. In the case of 

 one gasteropod, Patella, Hesse (:02 b ) has found a single fibril passing 

 from the brush-like ending in the rod directly through the slender pig- 

 mented, sensory cells and thence on into the neurite. Unfortunately, 



