2l6 Mary I sab ell e Steele 



tissue, each furnished by a different form. Sections taken from a 

 crayfish eye fixed sixty-two and a half hours after the operation 

 show a condition that is apparently an early stage in such develop- 

 ment. At this stage the network is not yet compact and is found in 

 chains of elongated cells, showing nuclei dividing amitotically. 

 These chains of cells run in all directions but do not appear to 

 develop from the hypodermis. Some sections show these chains 

 extending from the injured retinulae which still surround the 

 remains of the old cones (Fig. 74). This suggests that the old 

 retinular cells are undergoing rapid multiplication. 



The chains of cells found in the crayfish eye differ in the follow- 

 ing respects from the network of abnormal tissue found in Palae- 

 monetes viridis and hermit crab, shown in Figs. 17 and 25. In 

 the cases of the hermit crab. Fig. 25, and of Palaemonetes viridis, 

 Fig. 17, the cells constituting the network are no longer recog- 

 nizable as chains and the nuclei no longer stain deeply nor appear 

 to be dividing. These differences may be accounted for by the 

 following facts. First, the whole available space in the stump of 

 the eye was completely filled with the network in the eyes of hermit 

 crab and Palaemonetes viridis and the chains of cells had become 

 so completely interwoven that their original character could no 

 longer be recognized. Second, it is probable that the nuclei no 

 longer stain deeply because the cells have ceased active division. 

 It has been shown that the cells cease to divide actively soon after 

 the secretion of pigment begins. In these cells the secretion of 

 pigment had begun. 



Assuming that these apparent differences have been accounted 

 for we may now turn to their likenesses which suggest a similarity 

 of origin. The most suggestive likeness is that in each of the three 

 forms the abnormal tissue appears to have developed outward 

 from the base of the wounded area rather than inward from the 

 periphery. The most striking evidence of this is the fact that 

 masses of old pigment appear near the periphery as if they had 

 been carried outward by the growth of the new tissue. These 

 abnormal tissues, in the case of hermit crabs and Palaemonetes, lie 

 close against the cuticle and several layers of the cells are flattened 

 as if they were the oldest cells and had been pressed against the 



