70 T. H. MORGAN. 



It may not be out of place to say a few words about the forma- 

 tion of bundles of elements in the eyes of many Arachnids, 

 as the preceding may have some bearing on the question. In 

 several of the Arachnid median eyes the retinal elements group 

 themselves into bundles of a varying number of cells — the 

 retinulse. Watase has advanced the idea that these bundles 

 represent each an invagination — a pit — of ectoderm, and repre- 

 sent the process of formation of the eye elements — ommatidia. 

 He homologizes the retinulse of the simple eyes with the reti- 

 nulse of the compound eye ; in the latter he also believes they 

 represent ancestral invaginations. I do not care to carry my 

 comparisons into the compound eye, but shall speak only of the 

 simple eyes, in which, I think, there is not sufficient evidence 

 that these bundles each represent an invagination. 



It seems most probable that these groups of cells forming 

 retinulse represent secondary changes in the eye. If the Arach- 

 nid eyes have, as I believe, come along some such lines as have 

 been just sketched, then it is evident that the eyes have several 

 times undergone profound changes of which the median Arachnid 

 eye to-day is the outcome. The Insects' eyes show no such com- 

 plicated groupings of cells as do the Arachnid eyes. The Pycno- 

 gonid eyes show no trace of such bundles of cells. And by no 

 means do all the Arachnid eyes show this, as I have satisfied 

 myself in the Spiders. So, unless one had preconceived ideas to 

 apply to the simple eyes, I see no reason for supposing such a com- 

 bination of cells to represent an invagination. On such a theory 

 the clear central part of the bundles — the center being formed by 

 the union of the inner parts of the cells forming the group — 

 represents the original chitin secretion of the hypodermal cells. 

 I can find no evidence to support this assumption, although I 

 have examined carefully, both by sections and macerations, the 

 retinula-bundles of several Arachnids. The clear central part — 

 the rhabdome — is not a secretion from a cell, hut only a clear 

 protoj)lasmic (?) jyortion of it, and is included in the general wall 

 of the cell. Nor is there any evidence that it is of a chitinous 

 nature, for to micro-chemical reagents it acts entirely differently. 

 Further, an apparently similar substance is formed at the sides 

 of part of the retinal elements of both Insects and Pycnogonids, 

 and although in the former the cells are in the proper position, 



