MUSEUM OF COMPARATIVE ZOOLOGY. 47 



produced between old ones, but only on the edge of the ommatidial 

 area. 



The changes in the ommatidia themselves can be studied most readily 

 in longitudinal and transverse sections of the retina. Figure 47 is an 

 enlarged drawing of that portion of the retina which is marked by a 

 bracket in Figure 46. It will be noticed that in this stage, E, the nuclei 

 are limited for the most part to the distal half of the retina. The 

 middle of the retina is occupied by a band of pigment, which gradually 

 fades as it approaches the basement membrane. The space between the 

 basement membrane and the ganglion is relatively narrow, and is occu- 

 pied chiefly by nerve-fibres. Returning now to the nuclei in the distal 

 half of the retina, it is to be observed that the general arrangement 

 which was pointed out in the last stage also persists here. The nuclei 

 of the distal retinulse are still in circles of six (compare Figs. 47 and 

 48, nl. dst.), and are very close to the external surface of the eye. In 

 the centre of each circle is seen a round pink body, the tip of the cone- 

 cells (Fig. 48 con.). Directly below the level of the nuclei in the distal 

 retinulse are the pairs of nuclei belonging to the corneal hypodermis 

 (Figs. 47 and 49, nl. cm.). Each pair of corneal nuclei surrounds the 

 distal end of the cone. 



Below this level one meets the four nuclei of the cone-cells (Figs. 47 

 and 50, nl. con.). From the side view (Fig. 47) it will be seen that the 

 groups of cone-cells are spindle-shaped in outline, and have their nuclei 

 arranged in a transverse plane at the thickest part of the spindle. The 

 nuclei which in stage C formed the proximal band are scattered in this 

 stage between the deep ends of the cones (Fig. 47, nl. px.). They are 

 not definitely arranged. In order to estimate the number of nuclei in the 

 proximal band for each ommatidium, I counted these nuclei as seen in a 

 series of tangential sections. In the outermost section in which the prox- 

 imal nuclei occur there were six nuclei around each cone. These nuclei, 

 however, were arranged in circles similar to the circles of the corneal 

 nuclei ; consequently for each ommatidium there were only two nuclei 

 in each circle of six. The remaining nuclei were all embraced in the 

 two succeeding deeper sections. In each of these two sections there 

 were about nine nuclei around each cone. The arrangement of these 

 nuclei was extremely irregular, and it was consequently difficult to 

 estimate the number of nuclei which belonged to one ommatidium. 

 Most of the nuclei were situated between three cones, therefore, about 

 one third of the nuclei around a given cone can be considered as belong- 

 ing to the ommatidium represented by that cone. As there were in 



