52 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



I have found no evidence in these cells of the existence of an 

 axial fiber such as I have described for the prism and pyramid cells. 

 I find no definite arrangement of the nuclei of the retina into definite 

 layers, but the nuclei of the three kinds of cells lie quite mixed, 

 sometimes one kind lying deeper than the other as can be seen in 

 the figures. Again, they may lie quite at the same level. (This 

 point will be referred to later.) 



It is these long pigment cells that I believe retract their 

 pigmented part from between the prisms and pyramids when the 

 medusae are placed in the dark, protruding with their pigment 

 when placed in the light. Fig. 5 is a section from a slightly 

 pigmented retina killed in the dark. The parts of the cells pro- 

 jecting beyond the pigmented zone, and which would lie between 

 the prisms and pyramids (here not shown) of the vitreous body are 

 seen to be narrower than in sections from retinas killed in the light 

 (Figs. 1, 3, 4, 7) and the cells themselves appear in a condition of 

 retraction as is shown by their large centrad portions with the nuclei, 

 which latter, also, here lie at quite a lower level than the other nuclei. 

 (The pyramid cells were not shown in this series.) I occasionally 

 found appearances like Fig. 5 in retinas killed in the dark (indeed, 

 in some the pigmented portions in the vitreous body were much 

 thinner and more retracted than in Fig. 5). Yet this appearance 

 was not of sufficiently general occurrence to leave no doubt as to 

 its significance. As positive evidence, however, I cannot give it any 

 other interpretation than the one given that the cells retract 

 themselves with their pigment when in the dark. Again, it must 

 be added that the nuclei of these cells may occasionally lie quite 

 deep even in retinas killed in the light. Indeed, like structures in 

 different retinas may vary considerably in size and shape. None 

 of my darkness retinas, however, showed such a large proportion of 

 the pigmented parts of the long pigment cells projected between 

 the prisms and pyramids as did the light retinas. I examined 

 and tabulated all my series with respect to the extent the long 

 pigment cells were projected into the vitreous body, and I found 

 that those which showed these cells with their pigment least 

 projected between the prisms and pyramids to be those that had 

 been killed in the dark. I thus feel satisfied that the pigmented 

 parts of these cells become in part or quite completely retracted from 

 between the prisms and pyramids of the vitreous body when in the 



