parkek: eetinal pigment cells of pal^monetes. 287 



of auy kind of muscle. These observations have led me to conclude 

 that muscular action, as ordinarily understood, has nothing to do with 

 the migration of the distal retinular cells. Obviously, ciliary action is 

 in no way connected with the movements of these ceUs, and there is left 

 then only amoeboid movement as a means of explaining these changes. 

 Each distal cell might be compared to an amoeba, which in its migrations 

 outward and inward uses its processes to guide its general motion. The 

 rate and general character of the movement agree well with this expla- 

 nation. In one respect, however, there is disagreement. Herrick ('91. 

 p. 455), in his account of the action of the distal cells in Paleemonetes, 

 states that, on contracting, these cells fold together somewhat as a ribbon 

 might be folded transversely to its length (cf. Fig. 10), and he believes 

 that, on expanding, they unfold again. This condition is one not easily 

 reconciled with amceboid movement. 



Through the kindness of Professor Herrick I have had the privilege 

 of studying his preparations, and I can confinn his statement that in the 

 contracted condition (Figs. 9 and 10) the cells exhibit a series of trans- 

 verse folds, which are entirely absent from the expanded form. These 

 folds, however, occur, so far as I am aware, only in eyes which have been 

 kept an exceptionally long time in the dark (thirty-eight days in the 

 case of Professor Herrick's specimens), and are then exposed to the light. 

 In my own preparations, none of which had been kept in the dark more 

 than twelve hours, no trace of such folding could be discovered, and I 

 have therefore been led to regard these folds as abnormalities induced 

 by protracted retention in the dark. Xot with standing this interpreta- 

 tion of the folds, they throw important light, I believe, upon the normal 

 action of the distal retinular cells. 



The exact form of these folds is not so simple as might at first be 

 supposed. The body of each cell in its contracted condition consists of 

 an elongated thickened axial portion and two lateral wing-like expan- 

 sions, each of which terminates in a rather sharp edge. In other 

 words, these distal cells, when contracted, instead of assuming the 

 usual ovoid form, retain more or less the shape that they had when 

 expanded (Fig. 8). In a longitudinal section through the axial portion 

 of the body of one of these cells (Fig. 9) slight folds are observable. 

 In similar sections through the edge of the lateral wings (Fig. 10) the 

 folding is seen to be much more pronounced. The folds are most con- 

 spicuous at the edges of the wings, and lose in prominence toward 

 the axial part of the cell. Another peculiarity of these folded cells, 

 as compared with those kept a shorter time in the dark, is that 



