MUSEUM OF COMPAKA.TIVE ZOOLOGY. 75 



metamorphosis rather than a simple secretion. Of one thing, at least, I 

 am convinced, — the tapetum owes its origin to a limited number of cells, 

 the nuclei of which become very much elongated during the process of 

 involution. How this takes place can best be shown in connection with 

 a general account of the changes accompanying the formation of the eyes 

 which possess a tapetum. 



The hypodermal infolding in the eyes of the "post-nuclear" group was 

 not studied in detail by Locy ; it appears to be considerably more com- 

 plicated than in the case of the anterior median pair. This I have been 

 able to make out from the specimens which Mr. Locy has kindly placed 

 at my disposal.* Most of the figures on the accompanying plates are 

 intended to illustrate these co]iditions. The first seven figures (PI. I) 

 present the median faces of successive sagittal sections from the left half 

 of the head of an individual about four days after hatching. The first 

 section is the one nearest the median plane. The directions of the in- 

 foldings are such that sagittal sections are more favorable for the study 

 of the posterior median and anterior lateral eyes than for the posterior 

 lateral. The nature of the infolding-process is most readily understood 

 by the aid of sagittal sections of the posterior median eye ; and hence I 

 begin the description with that eye. 



Of all the sections studied, those which are represented in Figures 

 8 and 9 (PI. I) are in some respects the most satisfactory, hut in other 

 respects they are possibly misleading. There is a considerable thickening 

 of the hypodermis in two regions, and these two thickened tracts appear 

 to be connected by a continuous row of nuclei (tap.) so arranged as to 

 suggest that an S-shaped folding of the hypodermis has taken place. The 

 principal difference between this condition and that described by Locy 

 appears at first sight to be due to the relative thickness of the three com- 

 ponents of the ** S." In the anterior median eye the middle part is from 

 the beginning the thickest ; in the present case it is the thinnest. In one 



* This paper was begun in the belief that there was no important difference 

 in the method by which the pre-bacillar and the post-bacillar types of ocelli are de- 

 veloped. After a large portion of the paper was already written, the author received 

 (March, 1886) from Mr! Locy for re-examination the preparations which had served 

 as the basis of his paper. The results of the re-investigation of his material, al- 

 though not sufficiently complete to form an entirely satisfactory presentation of the 

 subject, are incorporated here because they are deemed of importance, and because to 

 have waited until the questions to which they give rise could have been more ex- 

 haustively studied would have necessitated both an extension of the paper beyond 

 the original plan and an undesirable delay in its publication. 



