MUSEUM OF COMPARATIVE ZOOLOGY. 77 



be traced in Fig. 2, although in this section the nuclei of the " band " 

 (r.) are more regularly polygonal. 



The further changes and the ultimate fate of each of these four tracts 

 seem fairly evident from a simple comparison of this figure (Fig. 2) with 

 Fig. 12, which shows a corresponding view of the same eyes (but from 

 the right side of the head) of an individual killed eight days after hatch- 

 ing. (Consult also Figs. 11, 16, 20-24, and the explanations of the 

 figures.) The relative positions of the parts have become slightly changed 

 in the later stage, owing to a continuation of the process of folding and 

 the closer approximation to each other of the three anterior regions. 



Numbering from behind forwards, it will be seen that the fourth or 

 last tract {p r. Fig. 12) has grown backward untU it now lies underneath 

 nearly the whole of the other three regions, and that the first tract 

 {pr r.) has grown forward in a corresponding manner, and thus intervenes 

 between the cuticula and the greater portion of the rest of the ocellus. 

 In the place of the third tract (tap.) the ** tapetum " now appears, with 

 here and there a greatly elongated nucleus, and in the second tract (r.) 

 the ends of the cells, which were previously directed forwards, and are 

 now directed downwards, — *'. e., toward the tapetum, — have developed 

 the bacilli (hac.) characteristic of retinal cells. 



From this stage onward, the significance of each of the four layers is 

 evident, and the determination of the homologies with the three layers 

 of the other type is to a certain extent possible. 



The first or posterior tract {pr r.) becomes the most superficial layer 

 and secretes the lens (Figs. 12, 22) ; it is the equivalent of the so-called 

 " vitreous body." * The cell-boundaries in this, as in the other layers, are 

 not made readily distinguishable by the process of preparation employed ; 

 but the shape and direction of the well-stained nuclei show that they are 

 quite oblique to the surface of the lens, and that some of them are 



* This layer of cells, which I have hitherto called " vitreous body " or " vitreous," 

 in conformity to the prevalent nomenclature, deserves a designation more in keeping 

 with its primitive function, — the secretion of a cuticular lens. Any designation 

 intended to replace so simple a word as "vitreous" must be equally brief in order to 

 be acceptable. I propose the name lentigen as a substitute for " vitreous body." 

 I believe this substitution is the more desirable since, according to the best present 

 information, there are probably some cases (c. g. Dytiscus) in ^fhich " lentigen " and 

 " vitreous body " would not be strictly identical. According to Grenacher's descrip- 

 tion, certain of the pre-retinal cells in Dytiscus do not abut upon the lens, and their 

 participation in its production may therefore be questioned. They do intervene 

 between the lens and the sensitive surface, however, and may appropriately retain 

 the title "vitreous" cells. 



