53 T. H. MORGAN. 



more highly magnified than in the preceding cases. The proto- 

 plasm runs down to the end of the cell, but on each side are the 

 rod-like bodies. On the left side of this element is a piece of a 

 rod of a neighboring cell ; the bacilli being formed by the fusion 

 of a pair of these rods of adjoining cells. Further, the end of the 

 retinal element has been so twisted that one also gets a side view 

 of the rod, showing it to be thinner through than broad. This 

 preparation is unsatisfactory, in that it is broken off at the point 

 where the cell-wall is continuous with the bacillus. 



In Fig. 44 we see a copy of a part of the superficially lying 

 retinal elements of the eye of Pallene. It shows how the cells 

 lie side by side in the superficial part of the middle layer, but at 

 one point one of the under cells comes out to the surface. The 

 nuclei are placed in the upper (outer) part of each cell, and below 

 these cells abut against those on the other side and form the 

 raphe (not seen here). All of the middle layer cells seem to be 

 very much alike, and although I have examined quite a number 

 of macerations, 1 have never obtained any decided departure 

 from those given above. On the other hand, I cannot deny that 

 other cells may exist in this layer, for, on account of the small 

 size of the eyes and the minute size of the retinal cells, it would 

 be very easy to overlook any unimportant elements, but I think 

 there can be no doubt but that what we have called the retinal 

 elements form the greatest part of the middle layer. 



If we next examine the cells of the inner layer — the pigment 

 cells — we find them showing a very great diversity of shape. 

 They are easily macerated apart by Haller's fluid (modified as 

 above). A few of these cells are shown in Figs. 45 a, b, c, d, 

 illustrating the main varieties. 



There seems to be a more or less definite distribution for these 

 different kinds of cells, and we may refer them, 1 believe, to 

 different parts of the posterior layer. Those pigment cells in the 

 innermost part of the eye, lying over and at the sides of the 

 raphe, are shown in Fig. 45 a. These are short, somewhat 

 cylindrical cells, about as high as wide, and fitting tightly into 

 one another. Ten of them form the mass in this figure, and in 

 some of them the nucleus can be seen, and in all the cell boun- 

 daries made out. The cells found over the sides of the eye show 

 great differences in size and shape, as seen in Fig. 45 b, c and d. 



