112 BULLETIN OF THE 
In 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.! 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 2, 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 zx 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 ¢wo, 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 be 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 any 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. 
