P. S. CONANT ON THE CUBOMEDUS^E. 55 



from each other more plainly. Curiously enough I have been unable to 

 demonstrate in Tripedalia the " visual fibres " of Schewiakoff. Here and 

 there were found spaces that at first sight reminded of them (Fig. 68, sh), 

 but they contained no central fibre, and were probably due to shrink- 

 age. The polygonal areas themselves, however, often contained a clear 

 spot in the centre, at one side of which would be found the cross-section 

 of the fibre, as is shown in many cases in Fig. 68. The clear spot is here 

 undoubtedly due to shrinkage of the gelatinous substance of the prism. 



I think that these prisms and fibres are the direct continuations of 

 retinal cells. In a section such as that drawn in Fig. 63, which takes 

 just the very tops of the cells of a slightly pigmented retina, in the 

 centre of the section just grazing the space that lies between the retina 

 and the shrunken vitreous body, most of the cells toward the middle 

 (where especially the extreme tips are taken) show in their centres a dot 

 exactly corresponding to the dots in the polygonal areas of the vitreous 

 body. In the exact middle of the section, where only the cell walls 

 appear, slightly indicated, a dot is seen in each case. The size and shape 

 of the ends of the cells correspond with those of the polygonal areas in 

 the vitreous body, and I do not doubt that the latter are continuations of 

 the former. The vitreous body, then, instead of being homogeneous, is 

 composed of the clear highly refracting outer ends of retinal cells. The 

 assumption lies near that these are the true visual rods, but of course it 

 is assumption only. 



To give a brief review, the points in which my conclusions differ 

 from those of Schewiakoff are as follows : I find (1) that the long pig- 

 ment streaks are parts of retinal cells continued into processes like his 

 visual rods ; (2) that the vitreous body is composed of prisms with central 

 fibres proceeding from retinal cells ; (3) that I am unable to get satisfac- 

 tory evidence of two types of cell distinguishable within the retina, and 

 at any rate find considerable evidence against the two types he distin- 

 guishes. 



These results are not wholly satisfactory, for they leave us with 

 three kinds of fibrous processes in the vitreous body which for the 

 present we are unable to trace to three, or even two distinguishable 

 types of cell in the retina. It would be more pleasing if we could 

 confirm Schewiakoff s simple conception of the structure, with its one set 

 of visual rods in the vitreous body referable to a clearly marked type of 

 sensory cells in the retina, but I think the evidence that has been brought 

 up justifies the conclusion that in some respects he saw too much, in 



