THE PYGNOGONIDS. 59 



In d the cells are drawn out at one side, becoming longer in one 

 diameter, and this process seems continuous into a nerve fibril, 

 though the process could only be traced for a short distance, and 

 I have not succeeded in tracing it actually into such a fibril. 

 In c two cells are shown which have fused together at the base, 

 and from the point of union a process runs out which presumably 

 is continuous into a nerve fibril. I think one must be impressed 

 by the close similarity between two such cells shown in c and 

 the similarly arranged retinal cells of Fig. 40. Now, although 

 I have no actual proof that the cells d lie nearer to the cells «, 

 and that the cells c are farther around to the sides of the eye, yet 

 from what I have seen I think it exceedingly probable that in 

 passing from the innermost cells of the pigment layer to the 

 outermost (those nearest the hypodermis) we go through a series 

 of changes as shown by a, d, c. Finally, in Fig. 45, b, there is 

 seen a long, slightly pigmented cell resembling much in shape a 

 retinal element. I cannot affirm positively that this cell belongs 

 to the pigment layer, but it seems exceedingly probable that it 

 does. If so we find quite a good series connecting the most 

 modified pigment cell on the one hand with the retinal elements 

 on the other ; and, further, each of those pigment cells seems to 

 connect by quite a large process with a nerve fibril. The 

 important bearing of this arrangement, etc., will be discussed in 

 another place. 



The Development of the Eyes of Tantsttlum. 



Instead of beginning with the earliest stages of the eye, which 

 are the most difficult to understand clearly, let us start at the 

 other end, when the eye has not quite reached maturity, and 

 work thence backwards. 



Fig. 51, Plate VIII, is from a longitudinal section through the 

 posterior eye of a larva which has just acquired four pairs of 

 walking legs. (See Fig. XVIII, Plate V.) In this the ecto- 

 derm has, in hardening, separated somewhat from its chitinous 

 covering, but there is no special thickening of the chitin over the 

 eye to form a lens. The three layers of the eye which were 

 found in the adult are here easily recognized. The outer layer, 

 which is formed but slightly modified hypodermal cells, con- 

 tains large nuclei, which are arranged in a single layer. The 



