76 H. HEATH :xnd M. H. SPAULDING, 



of pigment, but in several cases they have been traced to the pig- 

 mented retinal portion. 



Concerning the development of these organs there are certain 

 features that are remarkable and apparently unique. In the earliest 

 undoubted stages both regions of the eye, the lens and retina, are 

 already differentiated. In Fig. 13 two cells superficially pigmented, 

 constitute the retina while a few scattered cells represent the be- 

 ginning of the lens. Concerning the origin of these elements it is dif- 

 ficult to speak with full assurance. The cells of the retina, ex- 

 clusive of their pigment are in size and general character much 

 like some of the cells in the surrounding connective tissue, but 

 otherwise there are no cogent reasons for ascribing to them such an 

 origin. 



Very early in the development of the ocellus other cells (Fig. 14 

 s. c) than those of the retina and lens and intermediate in size migrate 

 from the surrounding (probably connective) tissue and take up po- 

 sitions external to the retinal cells. Their subsequent history shows 

 them continuing in the nature of ensheathing cells and finally forming 

 the walls of the lens. 



The nuclei of the lens are relatively very large and their chromatin, 

 in nearly all cases almost homogeneous, stains but faintly in Dela- 

 pield's haematoxylin. Usually the cytoplasm is relatively small in 

 amount, forming in many cases but little more than a film around the 

 nucleus. About the stage shown in Fig. 14 where the eye is half 

 developed some of the nuclei become characterized by a larger size, 

 by a strong affinity for logwood stains and by a granular appearance 

 of the chromatin. In succeeding stages of development they continue 

 to enlarge until their final volume is fully a hundred-fold greater 

 than at first. During this progressive change the chromatin swells 

 greatly until its maximum amount is reached at a stage when the 

 nucleus is about the size of the one shown in Fig. 15. Beyond this 

 point the chromatin rapidly loses its affinity for stains, becoming in 

 haematoxyhn light blue, a faint brown and finally a light yellow be- 

 fore it completely dissolves. In each lens upwards of eight or ten 

 cells pass through the transformations just described and form huge 

 vesicles in which no trace of cytoplasm is visible. These remain 

 groui)ed together and are surrounded by sheath cells whose position 

 can only be determined by their nuclei (Figs. 15, 16). In the final 

 stages the walls of the vesicles break down and ultimately almost 

 completely disappear. 



