smith: eyes of pulmonate gasteropods. 251 



of other cells into such shapes as Figure 28 (Plate 3) exhibits. The 

 largest seusojy cells of the eve (Fig. 28, which lacks the rod) are in the 

 accessory .retina. Such a cell may be compared with one from the chief 

 retina of the same eye (Fig. 30) near the cornea. 



The relation of each sensory cell to a group has already been men- 

 tioned ; such a group is shown in side view in Figure 9 (Piute 1). 

 Here a single, long, sensory cell occupies the axis of the group, and is 

 surrounded by four shorter pigment cells. The proximal halves of the 

 pigment cells, being attenuated, do not hide the enlarged, nucleated 

 portion of the sensory cell. 



Sections prepared in ordinary ways do not show any fibrillar structure 

 in the vicinity of the nucleus, neither is there in this region any trace of 

 fibrillae in macerated preparations, although Patten ('86) shows most 

 abundant fibrillae in the retina of Haliotis, "which he prepared by a 

 maceration process. By depigmenting the sections the cytoplasmic struc- 

 ture is either destroyed or rendered unintelligible, and the nucleus is 

 reduced to a homogeneous ball. In macerated preparations the nuclear 

 structures are probably not accurately preserved. However, the nucleus 

 of the sensory cell may always be identified by its large size, rounded 

 form, fine granulations, and especially by the presence of a large, highly 

 refractive nucleolus. In sections stained with haematoxylin the nuclei 

 of the sensory cells are always seen to be larger than those of the pig- 

 ment cells, and are very conspicuous on account of their large nucleoli 

 and abundant chromatic substance. 



Inside the optic capsule the neurites are gathered into strands, which 

 lie near the inner face of the capsule and converge to the region of 

 the optic nerve, where they pass through several holes in the capsule 

 (Plate 3, Fig. 35), and constitute the optic nerve. In one case nine of 

 these strands were visible in a cross section of the optic nerve at its emer- 

 gence from the capsule. These strands remain more or less separate from 

 one another for a little way because of the connective tissue surround- 

 ing each, and they show a tendency, — first noticed under low powers, — 

 even at some distance from the eye, to group themselves in the form of 

 hollow cylinders (Fig. 22). Figures 22 and 35 are from methylen-blue 

 preparations, and show, in the latter figure, approximately the number 

 of neurites in a strand, in the former, the total number of neurites in the 

 nerve. The neurites, it is seen, are very small compared with the axis- 

 cylinder of vertebrates. The sheath of the nerve, which is continuous 

 with the optic capsule, is about as thick as the latter, except at the 

 point where the nerve arises; here it is thickened. Within the sheath 



