204 JAMES ROLLIN SLONAKER 



also produce modifications in structure. For example, it may 

 be said that the eye of birds is characterized by the presence of 

 a fovea for clear and distinct vision. The hen, so far as I know, 

 is the only bird which does not possess a fovea, while the nearest 

 related wild forms examined do (Slonaker, '97). Could the 

 absence of a fovea in the hen have been brought about by domes- 

 tication? If so, other structures may likewise have been 

 modified. 



The eye of the adult sparrow has been previously described 

 (Slonaker, '18). This paper will deal mainly with macroscopical 

 descriptions of the developing structures until they have reached 

 the adult condition. 



The material was secured from various nests. The eggs were 

 thus in various stages of incubation. Eggs which were obviously 

 fresh were placed in an incubator. In this way the size of the 

 embryos at different ages was ascertained. The material which 

 was collected in various stages of incubation was readily classified 

 by comparison with this data. 



The earliest stages of development of the sparrow^ were not 

 secured. In order to show the early development of the eye of 

 the bird, I have used the chick for the stages I failed to secure in 

 the sparrow. This, I think, is permissible, since the eyes of all 

 birds follow the same order of development. 



All embryos were hardened in Perenyi's fluid and imbedded 

 in celloidin. Serial sections were made and stained in haema- 

 toxylin and eosin. Perenyi's fluid preserved the retina in the 

 most perfect condition of any hardening fluids tried, and at the 

 same time it decalcified the bone wherever present. The sections 

 of the embryos were made in a horizontal plane parallel to the 

 lines S-S as shown in plate 1. In some of the older embryos the 

 eyes were sectioned in both horizontal and vertical planes parallel 

 to the axis of vision. Camera-lucida drawings were made 

 wherever possible. All other drawings were made to scale. 



