smith: eyes of pulmonate gasteropods. 2-47 



in the retina of Limax, Henchman ('97) reached the correct conclusions ; 

 but, as Hesse (:02 b ) intimates, only the study of the question by means 

 of intra vitam staining or silver impregnation — in the use of which no 

 one has heretofore succeeded — can definitely settle the nature of the 

 cells of the retina. 



It is, therefore, a pleasure to have succeeded in the use of methylen 

 blue (see p. 239) on two species of pulmonates, namely, Limax maxim us 

 and Helix pomatia. The evidence from this method will be presented 

 in detail in connection with the account of the sensory cells. I desire 

 now only to point out that methylen blue does not stain the pigmented 

 cells of the retinas of these two species, if the staining is checked while 

 the tissues are still alive. Sections of methylen-blue preparations show 

 only the unpigmented cells impregnated, whereas the pigment cells are 

 not only unstained but also in their proximal part so colorless that even 

 their nuclei are usually invisible. 



In the retina of Helix the pigment cells are essentially like those of 

 Limax, except that their nuclei are a little larger. 



The sensory cell of the retina of Limax (Plate 1, Figs. 4, 9 ; Plate 2, 

 Fig. 14) has the general form of a long-necked flask. The enlarged end, 

 which contains the nucleus, is proximal and turned toward the capsule ; 

 the neck-like portion points toward the lens and pierces the pigment 

 zone, distal to which it is differentiated as the rod. 



Typically the proximal end of the cell gives rise to two or more proc- 

 esses. All but one of these processes are radiculae — a term which 

 Grenacher ('86) applied to the special organs of attachment which he 

 found in the retina of a heteropod. Near to the cell body the radiculae 

 are not easily distinguished from the neurite, except in some cases by 

 their direction ; but farther away from the cell near the capsule they 

 fail to be stained by methylen blue as intensely as the neurite. At the 

 capsule they branch and pass into its meshes, where they branch again 

 repeatedly like a flat root-system of a plant. I have seen the branches 

 in the capsule only in methylen-blue preparations, in which they appear 

 more or less refractive and faintly blue, or slightly greenish, depending 

 upon whether they are well stained or almost colorless. That these 

 branches are not connective-tissue fibres is evident from their size, which 

 is much greater than that of such fibres; many connective-tissue fibres 

 are impregnated by over staining, and so comparisons are easy. There 

 is no doubt, then, that the sensory cells, as well as the pigment cells, are 



