MUSEUM OF COMPARATIVE ZOOLOGY. ae 
in the retinas of embryos; and, although the cells may possibly be 
intrusive, the evidence on the whole favors the view that they are 
ectodermic in origin. 
Several functions have been attributed to the hyaline cells. © Their 
close connection with what Beddard took to be the proximal extension 
of the rhabdome led him (’88, p. 450) to suspect that they might be 
rudimentary retinular cells, but, as he (p. 451) further remarks, the fact 
that no nerve fibres are connected with them opposes this view. Their 
transparency suggested to him (’84*, p. 22) that they might form a part 
of the dioptric apparatus; but it is difficult to understand, consider- 
ing their position, precisely what that function would be. I am inclined 
to believe, with Watase (’90, p. 293), that they are chiefly concerned 
with the support of the structures occupying the basal portion of the 
retina. 
In the retina of S. Schythei many of the open spaces between the 
cones and the basement membrane contain free non-pigmented cells 
(Fig. 61, cp. sng.). These have a distinct nucleus, finely granular pro- 
toplasm, and a sharply marked outline. On account of the extreme va- 
riations in form which the different cells present, it is probable that when 
living they exhibited amceboid motion. In appearance they correspond 
exactly to the blood corpuscles of the body spaces, and as they occur not 
only in the retina, but also in the rather large openings through the 
basement membrane (compare Fig. 64), and in the space proximal to 
this membrane, I am of opinion that they are blood corpuscles. 
The peculiarities which have led me to consider the ommatidium 
in Serolis separately from that of other [sopods, are two: the posses- 
sion of one or more hyaline cells, and the presence of only four 
retinular cells. The latter peculiarity, as I have already shown, is not 
fully established ; for in this genus, as in many other Isopods, the om- 
matidium really contains six cells, although two of these, the distal ones, 
are probably no longer functional as nervous structures. The other 
peculiarity, the possession of hyaline cells, is not a very important char- 
acteristic, for, as Beddard (’87, p. 235) has shown, these cells also occur 
in Aiga; and it is probable, moreover, that they must be regarded as 
abnormally enlarged elements, specialized from among those cells which 
in other Isopods fill the spaces between the ommatidia. What dis- 
tinguishes the ommatidium in Serolis from that of other Isopods is, 
therefore, not so much the possession of hyaline cells as the fact that 
its retinular cells are differentiated into two sets, proximal and distal. 
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