MUSEUM OF COMPARATIVE ZOOLOGY. 13 



retinuke, the most striking difference noticed in the cone-cells is a dimi- 

 nution in their diameter. (Compare Figs. 1, 6, and 8.) On a level with 

 the distal ends of the proximal retinulse, the groups of cone-cells still 

 retain their four-parted character (Fig. 9, cl. con.). Each group is easily 

 traced between the retinulse till it approaches the distal end of the 

 rhabdome. The change which here takes place is represented in Fig- 

 ure 12. Of the four ommatidia which are shown in this section, the one 

 indicated at a is cut slightly above the rhabdome. In the case of om- 

 matidia b, c, and d, the plane of section passes through the end of the 

 rhabdome. In each of these three, it will be noticed that the rhabdome 

 is surrounded by four bodies, which correspond in position to the four 

 cone-cells. The four ommatidia which are drawn in Figure 12 are in 

 no way exceptional, but represent a very usual condition. Many such 

 cases have been examined, and whenever the tip of the rhabdome was 

 in the section, it was invariably surrounded by the four bodies previously 

 mentioned. When, on the other hand, the plane of section did not pass 

 through the rhabdome, only the four cone-cells were present. The 

 round bodies at the sides of the rhabdome can be traced from section 

 to section, and I therefore believe them to be fibres. Moreover, there 

 is no break observable in their continuity with the cone-cells, and I 

 therefore further believe that they are the fibrous prolongations of the 

 cone-cells. They have one peculiarity which is worthy of comment. 

 As the cone-cell passes over into the fibre, a considerable diminution in 

 its diameter takes place. This is accomplished at the distal end of the 

 rhabdome, and within a space equal to the thickness of one or at most 

 two sections (7.5-15 //,). Occasionally there is to be seen a group, in 

 which one or two cells have been reduced to fibres, and the remaining 

 ones are as large in transverse section as an individual in ommatid- 

 ium a (Fig. 12). The conclusions which are arrived at from the study of 

 sections are confirmed by isolation-preparations. Figure 28 represents 

 a portion of an isolated group of four cone-cells from a single omma- 

 tidium. In the distal part of the specimen the four cells are intimately 

 bound together, but at the proximal end they appear as four separate 

 fibres. As in the transverse section (Fig. 12), the continuity of the 

 fibrous and thicker portion of the cone-cell, and the rapid reduction of 

 the cone-cell to form the fibre, are plainly seen. The cone-cells are usually 

 somewhat separated before they reach the rhabdome. It can scarcely 

 be said that they touch the rhabdome, although this is the region in 

 which they are nearest to it. As the fibres pass into the deeper part of 

 the retina they are found to lie nearer the periphery of the ommatid- 



