158 BULLETIN OF THE 



dermis ; they are, moreover, round in outline, while the other nuclei are 

 usually somewhat elongated. The same features can be observed in 

 tangential sections (Fig. G). Here, however, the outlines of the larger 

 nuclei no longer appear oval, since these nuclei are now cut in a 

 plane at right angles with their elongated axes. The nuclei in the 

 hypodermis which adjoins the retinal thickening resemble the larger 

 oval nuclei of the thickening. Nowhere in the adjoining hypodermis 

 have the granular masses with their smaller nuclei been observed. It 

 is therefore clear, that in C. pellucidus the retinal hypodermis is dis- 

 tinguished from the neighboring hypodermis, not only by its greater 

 thickness, but also by the fact that it is composed of two kinds of sub- 

 stance, each with its special form of nucleus. Since the protoplasmic 

 material of the retinal region contains nuclei which resemble those of 

 the surrounding hypodermis, it is probable that this material represents 

 hypodermis which has remained unmodified after the differentiation 

 of the granular bodies. As shown in Figure 3, the granular bodies are for 

 the most part limited to the deeper portion of the retinal thickening, and 

 the oval nuclei occupy the more superficial part. If these oval nuclei 

 represent undifferentiated hypodermal cells, it is only natural that they 

 should occupy a superficial position, for it is there that the function of 

 such cells, namely, the secretion of cuticula, could be most advanta- 

 geously carried on. In tangential sections of the retinal thickening, 

 both the nuclei of the undifferentiated hypodermis and the outlines of 

 the cells to which they belong are distinguishable (Fig. 5). These cells 

 when compared with those from the hypodermis of the sides of the stalk 

 (Fig. 4) are seen to be much smaller than the latter. Like those from 

 the sides of the stalk, however, they present no definite grouping. 

 This accords with the fact that the cuticula presented no special mark- 

 ings, such as facets, etc., for such markings could of course result only 

 from some special grouping of the secreting cells. 



It is difficult to say what the granular bodies with their contained 

 nuclei are. Doubtless they represent some element in the retina of the 

 functional eye reduced by degeneration to this form. The ommatidium 

 or structural unit in the retina of a crayfish consists of five kinds of 

 cells. . These are as follows : first, two cells in the corneal hypodermis, 

 lying next the cuticula ; second, four cone-cells directly below the 

 corneal hypodermis ; third, two pigment-cells, the distal retinulse, 

 flanking the cone-cells ; fourth, seven pigment-cells, the proximal reti- 

 nuhe, surrounding the rhabdome ; fifth, a few yellowish accessory pig- 

 ment-cells limited to the base of the retina. Excepting the accessory 



