PARKER: RETINAL PIGMENT CELLS OF PALZEMONETES. 287 
of any 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 cells, 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 Palamonetes, 
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 amoeboid movement. 
Through the kindness of Professor Herrick I have had the privilege 
of studying his preparations, and I can confirm 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. Notwithstanding 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 later»! 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 
