14 BULLETIN OF THE 



ium. They lie between the proximal retinulae, but at some distance 

 from the rhabdome (Fig. 14, cl. con.). They still retain, however, the 

 same relative positions in the ommatidium. Their peripheral location 

 is maintained (Fig. 17) till they are very close to the basement 

 membrane. 



The changes which the fibres undergo as they approach the basement 

 membrane is shown in Figure 19. In this section the plane of cutting 

 was slightly oblique to the basement membrane. Of the three omma- 

 tidia which are here represented, those indicated at c and d are cut at 

 about the same level, and very close to the basement membrane. Om- 

 matidium b is cut farther from the membrane than either c or d. The 

 fibres of the cone-cells are closer to each other in c and d than in b. 

 As this condition is generally true in other sections, it follows that, as 

 we approach the basement membrane, the fibres of the cone-cells con- 

 verge. The convergence is also shown in Figure 21. Of the omma- 

 tidia here figured, b is cut farthest from the basement membrane. In 

 it the four cone-cells {cl. con.) can be seen, and between them a dot 

 which represents the proximal end of the rhabdome (rhb.). Nearer the 

 basement membrane is ommatidium a, in which the four cone-cells can 

 be recognized, and to one side a fibrous area. The rhabdome does not 

 extend as deep as this. The fibrous area represents a region in which 

 the plane of section passes through an elevation on the distal face of 

 the basement membrane. Ommatidium d is still nearer the membrane. 

 The cone-cells are here brought more closely together, and are sur- 

 rounded by the fibrous substance of the elevation. The form of the 

 elevation is now seen to be that of a cross. At x is shown a basal section 

 of the cross-shaped elevation surrounded by four large openings through 

 the basement membrane. The cone-cells are no longer visible at this 

 level, and I therefore believe that without penetrating the basement 

 membrane they terminate in these elevations. This belief is further 

 supported by the fact that in transverse sections of the basement mem- 

 brane the cone-cells distinctly end in the substance of these elevations 

 (Fig. 29, cl. con.). 



The facts obtained from a study of the lobster's eye support the claim 

 made by Schultze and Grenacher, that the cone-cells and rhabdomes are 

 separate structures. In the case of the crayfish, Schultze, moreover, 

 saw the prolongations of the cone-cells, and traced them into the deeper 

 part of the retina. It is probable that in the crayfish, as in the lobster, 

 the fibrous ends of the cone-cells terminate in the basement membrane, 

 but this Schultze did not see. Such an omission is by no means sur- 



