22 BULLETIN OF THE 



against its sides (Fig. 14). Below its middle transverse plane the 

 rhabdome becomes gradually smaller and smaller, till finally it termi- 

 nates about 15 fx from the basement membrane. As the rhabdome con- 

 tracts in size, the retinulae enlarge. (Compare Figs. 14 and 17.) 



As I have already mentioned, the retinulae are definitely arranged 

 around the rhabdome, and this arrangement persists nearly to its 

 proximal termination ; but between the end of the rhabdome and the 

 basement membrane the retinulae rearrange themselves. This re- 

 arrangement of the retinulae is a step preparatory to their passing 

 through the apertures in the basement membrane, the general struc- 

 ture of which has already been described. It will be remembered that 

 under each ommatidium the distal face of the membrane presents a 

 cross-shaped thickening, and that in each of the four angles which the 

 arms of the cross make with each other there is an opening (Fig. 21). 

 The openings are oval in outline, especially on the distal face of the 

 membrane. One end of a given oval lies in the angle of the cross, 

 and the crosses are so close to each other that the other end of the 

 same oval lies in the angle of a neighboring cross. Each opening then 

 lies in the angles of two adjoining crosses, and through it pass two 

 groups of retinulae, one from each of the two ommatidia to which the 

 crosses correspond. 



The four groups into which the retinulae of a single ommatidium 

 are divided pass one through each of the four surrounding apertures. 

 Three of the groups consist of pairs of retinulae ; the fourth group is 

 represented by only a single retinula. Although these groups agree 

 numerically with the groups of retinulae, which, as I have already 

 shown, surround the rhabdome, they are not composed of the same 

 individual retinulae. 



For convenience of comparison, numbers can be assigned to the dif- 

 ferent retinulse. This has been done in ommatidium c (Fig. 15), where 

 the large odd retinula is numbered 1, and the remaining retinulae, pro- 

 ceeding in a circle to the left, are numbered 2 to 7. On this plan of 

 numbering, the four groups of retinulae which have been already indi- 

 cated as surrounding the rhabdome are composed as follows. What 

 may be called the first group is formed of retinulae 2 and 3, the second 

 group contains retinulae 4 and 5, the third retinulae 6 and 7, and the 

 fourth retinula 1. It will be observed (Fig. 15) that the seven retinulae 

 are also divided into four other groups by the fibres of the cone-cells. 

 Three of these groups are composed of pairs of retinulae, the individuals 

 of which lie nearest the angles of the rhabdome. In Figure 15, omnia- 





