MUSEUM OF COMPARATIVE ZOOLOGY. 45 



Crustaceans the nerve-fibres are always attached to the proximal ends of 

 the retinulae, it can be argued that the retina in this group has never 

 been inverted, but retains its original position, and that any structural 

 plan which involves the inversion of the retina is therefore probably 

 wrong. 



The Differentiation of the Ommatidia. 



In the development of the compound eye in the lobster, the deposi- 

 tion of pigment and the differentiation of the ommatidia take place 

 at about the same time. These changes occur at stage C. (Compare 

 Figs. 39, 41, and 42.) At this stage (Fig. 41) it will be observed that the 

 retinal layer is thickest at the lateral margin of the disk (the extreme 

 left in Fig. 41). The retina becomes thinner as one proceeds from the 

 margin toward the median plane (from left to right in the figure). 

 The thickest part of the retina, it will be recalled, was the part first to 

 be separated from the ganglion by the intercepting membrane. As it 

 was the first part of the retina to be separated, so it is the first part in 

 which the ommatidia are differentiated and pigment is deposited. 



The first steps in the differentiation of the ommatidia are seen in 

 Figure 42. Here the nuclei in the thicker part of the retina have sep- 

 arated into two bands, one distal (?/). and one proximal (x). The distal 

 band, as I shall presently show, can be further separated into a super- 

 ficial and deep layer. These two layers are close to the outer surface 

 of the retina, and approximately parallel with it. 



The arrangement of the nuclei which make up the distal band is most 

 easily observed when the retina is viewed from the surface. The group- 

 ing of these nuclei, from the time when the ommatidia are differentiated 

 to the adult condition, is so characteristic that the fate of the individual 

 nuclei can be easily traced. For the sake of simplicity I shall therefore 

 designate the different nuclei, from their first appearance in groups, by 

 the names of the cells in which they are ultimately found, although it 

 is to be borne in mind that in the early stages the cell walls are not as 

 yet differentiated. 



Viewed from the surface, the superficial layer of the distal band of 

 nuclei presents the appearance which is shown in Figure 43. In this 

 layer there are two kinds of nuclei, one elongated, the other roundish. 

 The elongated ones are always in pairs. They ultimately become the 

 nuclei of the corneal hypodermis. These hypodermal nuclei arise from 

 among the superficial nuclei of the distal band, and do not originate, as 

 Patten believed ('86, p. 645), from a fold which grows over the retinal 



