20 BULLETIN OF THE 



each group of cone-cells. In many cases the membrane has not been 

 observed. It was noticed by Newton ('73, p. 328), and as it is non- 

 cellular it is probably a feeble representative of what Herrick ('86, p. 44) 

 has described as a " chitinous " framework in the deeper part of the 

 retina in Alpheus. 



The Proximal Retinuloe. 



The proximal retinulee are pigment-cells which closely invest the 

 rhabdome. With the brownish accessory pigment-cells they constitute 

 the proximal band of brownish black pigment on the distal side of the 

 basement-membrane (Fig. 26, pig. px.). In some cases they appear to 

 terminate distally in rounded knobs, each of which contains a nucleus 

 (Fig. 1, rtn'. px.). In other instances, and these are of frequent occur- 

 rence, their distal ends, in addition to having a swollen nucleated part, 

 are prolonged into delicate fibres (Fig. 30). These fibres when present 

 extend toward the outer surface of the retina, and are applied, not to 

 the cone-cells, but to the fibrous portion of the distal retinulae. The 

 fibres have been traced only a short distance beyond the rounded ends 

 of the cells from which they originate. As the region into which they 

 extend is one readily studied in both sections and maceration prepa- 

 rations, and as these methods of study have given no evidence of fibres 

 other than that of the very short ones already mentioned, it seems fair 

 to conclude that the distal retinulee terminate as fine fibres a short dis- 

 tance in front of their nucleated portions. 



In transverse sections the distal retinulee first clearly appear in the 

 plane represented in Figure 9. Here each group of four cone-cells is 

 surrounded by a circle of seven retinulse. The section from which this 

 figure was drawn is in a slightly oblique plane. In moving from right 

 to left, one passes into deeper and deeper regions. In the more super- 

 ficial part of the section, the right-hand half, each retinula contains a 

 nucleus, which is surrounded by a small amount of pigmented cell-sub- 

 stance. In the deeper part of the section, the left-hand half, the plane 

 is below the region of most of the nuclei, and one sees the seven reti- 

 nulse densely filled with pigment. In the next section (Fig. 10), the 

 retinulaa are broader in transverse section. In their expansion they 

 have so far encroached on the space which they surround that it is 

 only large enough to allow the passage of the four cone-cells. The con- 

 traction of the space within the circle of retinulae takes place almost 

 in one plane, as can be seen in the longitudinal section (Fig. 1). In the 

 plane of Figure 10, the retinulse show a tendency to group themselves. 



