112 BULLETIN OF THE 



la the distal portion of the retinula in Cambarus there are eight 

 nuclei. The arrangement of these, as seen in successive transverse 

 sections, is shown in Plate X. Figs. 118 to 122. In Figure 118, which 

 represents the most distal section of the series, there are four nuclei, 

 and these are so arranged that there is evidently one for each omma- 

 tidium. 1 In the next section (Fig. 119) there are seven nuclei, none 

 of which were seen in Figure 118 ; the place for an eighth is indicated 

 by an open area, and the eighth nucleus itself is seen somewhat out of 

 place in Figure 120 (x). Four of the eight nuclei belonging in Figure 

 119 are arranged in a manner similar to those in the preceding sec- 

 tion, but are not to be confounded with them. The remaining four 

 are so placed that there are two for each ommatidium. Hence in this 

 plane there are, as a whole, three times as many nuclei as there are 

 ommatidia. In the next section (Fig. 120), omitting the nucleus 

 marked x, which has been recorded as belonging to the preceding 

 section, there are four nuclei, so arranged that there is one for each 

 ommatidium. In the following section (Fig. 121) the nuclei, omit- 

 ting the one marked x, which will be considered as belonging to the 

 next following section, are so arranged that there are two for each 

 ommatidium. In the last section (Fig. 122), the nuclei are not so 

 regularly grouped as in the previous section, but when taken with the 

 nucleus marked x in Figure 121, they constitute a group of four, the 

 arrangement in which is such that each nucleus is intermediate between 

 four groups of cone cells rather than between two, and therefore in the 

 plane of this section there is one nucleus for each ommatidium. From 

 this enumeration it is evident that the total number of retinular nu- 

 clei is eight ; namely, one in the first section, three in the second, one 

 in the third, two in the fourth, and one in the fifth. The structure 



1 The nuclei shown in Figures 118 to 122 are arranged upon either the plan 

 shown in Figure 118 or that in Figure 121 (omitting nucleus x). Imagine the 

 arrangement in Figure 118 extended over a large surface. The groups of four 

 cone cells could then he regarded as forming lines in the direction of the length 

 of the plate. These lines would alternate with lines of nuclei, and as the nuclei 

 in an}' line would alternate with the groups of cone cells in an adjoining line, the 

 number of nuclei must equal exactly the number of groups of cone cells ; i. e. in 

 this arrangement there is one nucleus for each ommatidium. In a similar way, 

 alternating vertical lines may be constructed from the arrangement in Figure 121. 

 One line would be composed entirely of nuclei situated one opposite each group 

 of cone cells ; the other, of alternating nuclei and groups of cone cells. In the 

 former, as well as in the latter, there would be as many nuclei as groups of cone 

 cells. Hence, in this arrangement the nuclei are twice as numerous as the groups 

 of cone cells ; i. e. there are two nuclei for each ommatidium. 



