MUSEUM OF COMPARATIVE ZOOLOGY. 103 



expect it to be composed of two, instead of four parts. Its position, bow- 

 ever, is one which is mure frequently occupied in other Crustaceans by 

 a slender extension of the cone cells than by a process from the rhab- 

 dome, and, notwithstanding its division into four parts, I am inclined 

 to agree with Grenadier, and to regard it as belonging to the cone cells 

 rather than the rhabdome. 



The distal retinular cells in Mysis surround the lateral faces of the 

 cones (Fig. 73, cl. dst). Apparently they reach the cuticula : their 

 proximal ends are attenuated and become lost in the region of the 

 nuclei of the proximal cells. Their pigment is limited to their proximal 

 halves, and consists of a distal layer of brownish material, proximal to 

 which is a much more extensive deposit of blackish granules. Each cone 

 is surrounded by six of these cells, as can be seen from their outlines 

 (Fig. 78, cl. dst.), and still more satisfactorily from the arrangement of 

 their nuclei (Fig. 75, nl. dst.). Each cell, however, participates in three 

 circles ; consequently, there are only twice as many of these cells as 

 ommatidia. 



The axis of each distal retinular cell is occupied by a transparent 

 rod, which in transverse section has the appearance of a light spot 

 (Fig. 77). In depigmented sections stained with Kleinenberg's hema- 

 toxylin, these rods are deeply colored (Fig. 78). I shall recur to their 

 probable significance. 



The pigment which is found in the region of the rhabdomes in Mysis 

 is of two kinds : blackish granules, and a fine flaky material, white by 

 reflected light, yellowish by transmitted light. The black granules are 

 for the most part contained in the retinular cells. The lighter pigment 

 is always associated with certain nuclei, two of which are shown in 

 Figure 90 (nl. ms'drm.). These nuclei are closely invested by the pig- 

 ment, and probably belong to the cells in which the pigment is con- 

 tained. 



The source of the yellowish pigment cells is not easily determined. 

 Apparently they are not limited to the retina, but also occur in the 

 spaces below it. At least these spaces contain masses of pigment and 

 nuclei which in all essential respects are similar to those distal to the 

 membrane (compare the two nuclei, nl. ms'drm., Fig. 90). In one case 

 the nucleus of one of these cells was found apparently caught in its 

 passage through an opening in the basement membrane (Fig. 91). For 

 these reasons I believe that the yellowish pigment cells on the two sides 

 of the membrane have had the same origin. The question as to the 

 source of the yellowish pigment cells in the retina, therefore, appears 



