252 bulletin: museum of comparative zoology. 



the connective tissue, except for an occasional nucleus, does not stain. 

 It may be imagined as filling the spaces between the neurites. To 

 impregnate only the neurites, however, it is necessary to stop the stain- 

 ing in time, otherwise the connective-tissue fibrils also will be stained, 

 and thus cause confusion. 



Except for the size and shape of the rod, the sensory cell in the retina 

 of Helix agrees in every way with that of Limax, as I have described it. 

 Although the proximal part of the sensory cells of Helix stains very 

 successfully with metbylen blue (Plate 3, Figs. 22-24, 26, 27, 29, 33, 

 34), the rods have not been stained. However, cross sections through 

 the narrow part of the cell and the distal ends of the pigment cells show 

 the sensory cell stained intensely blue. Figure 23 shows three such 

 cells siuTounded by pigment granules. The largest cell has a five- 

 pointed outline, because, lying nearer the chief axis of the eye, the cell 

 has been cut at a more distal level than the others, just as it begins to 

 expand into the rounded rod. For a longitudinal view of the rod and 

 sensory cell, see Figure C (p. 237), which is copied from Hesse. The 

 other two cells in Figure 23 agree with what was seen in Limax (Fig. 5), 

 except that the cell membranes are not visible. 



Since it will be necessary to refer repeatedly to the accessory retina, 

 and since it has not been adequately described by other writers, 1 desire 

 to recur to it again. The relation of the accessory retina to the cornea 

 is seen in Figure 13 (Plate 2), which shows a section made at right 

 angles to the chief axis of the eye and very near the proximal margin of 

 the cornea. The lens hides the centre of the comea (cm.), whose long, 

 clear cells — polygonal in cross section — are arranged around it in a 

 nearly radial fashion. The small, ellipsoidal nuclei of the corneal cells 

 occupy the basal ends of the cells and thus lie near the capsule. This 

 section of the accessory retina more neaidy reproduces all the parts of a 

 complete eye than any other example which I have seen. There are 

 parts of fifteen sensory cells in this one section. According to Hesse 

 the number of these varies from ten to fifteen. Besides the indifferent 

 cells, which, however, have no pigment, there is also a small amount of 

 highly refractive substance (Ins'.) which has the same consistency as the 

 chief lens, and, in fact, is accompanied by a small amount of vitreous 

 substance. Thus this retina, except that it does not have an optic cap- 

 sule of its own nor a separate nerve nor pigmented cells, might be called 

 an accessory eye. I have not positively determined whether or not the 



