48 BULLETIN OF THE 



these two sections about eighteen nuclei around each cone, it follows 

 that one third of this number, or six, represents approximately the num- 

 ber of nuclei for each ommatidium. If then the deeper sections con- 

 tain six nuclei for each ommatidium and the outermost section two, the 

 total number of proximal nuclei for each ommatidium must be eight. 

 I do not mean to imply that this estimate can be insisted upon as abso- 

 lutely invariable ; but I wish to show that, as these nuclei represent the 

 proximal retinulse in an adult lobster's eye, and as there are eight 

 such retinulse and about eight of these nuclei to an ommatidium, the 

 change which the eight embryonic cells undergo in becoming adult 

 retinulae is chiefly that of arrangement, and certainly does not involve 

 any considerable increase in numbers. 



At stage E, which was the one last described, the young lobster was 

 about to escape from the egg-shell. The next stage, F, is that of a 

 lobster about one inch in length. At this stage the optic lobes are 

 represented by optic stalks, and the distal rounded end of each stalk is 

 occupied by the retina. Figure 51 represents a longitudinal section of 

 a single ommatidium from this stage. The distal end of the ommatid- 

 ium is covered with a well developed corneal cuticula. The cuticula 

 is marked out into hexagonal corneal facets (Fig. 52). The facets of 

 course indicate the arrangement of the ommatidia. This arrangement 

 at the first differentiation of ommatidia was such that hexagonal and 

 not square facets w r ould have resulted if a cuticula had been then 

 produced. 



Directly below each corneal facet is a pair of crescentic nuclei, those 

 of the corneal hypodermis (Fig. 53, nl. cm.). These nuclei have in all 

 preceding stages shown a tendency to become elongated and crescentic 

 in outline, but it is in this stage that this peculiarity reaches its highest 

 development. Between each pair of hypodermal nuclei the distal end 

 of the cone-cells is usually seen (Fig. 53, con.). The four cone-cells 

 with their nuclei occur immediately below the corneal hypodermis 

 (Figs. 51 and 54, nl. con.). The cone itself is already formed in part, 

 and lies below the nuclei of the cone-cells. Proximally the four cone- 

 cells can be traced to near the middle of the retina ; here they are no 

 longer distinguishable. The proximal end of the cone itself terminates 

 as four processes, one on the outer lateral wall of each cone-cell (Fig. 56). 

 Surrounding the cone about midway on its length are the nuclei of the 

 distal retinulae (Figs. 51 and 55, nl. dst.). In transverse sections of 

 stage F, these nuclei show the same grouping in circles of six as they 

 showed in previous stages. The outlines of the retinulse are not visible. 



