MUSEUM OF COMPARATIVE ZOOLOGY. 103 
expect it to be composed of two, instead of four parts. Its position, how- 
ever, is one which is more 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 Grenacher, 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. Hach 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 
