MUSEUM OF COMPARATIVE ZOOLOGY. 97 



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, qy. 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 difiierent cells present, it is probable that when 

 living they exhibited amoeboid 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 Tsopods, 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 -^ga ; 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. 



VOL. XXI — NO. 2. 7 



