Jeffries.] 226 [February 7, 



cells and the mucous cells, very much like those of the rest of 

 the body. 



By the sixteenth day, in some cases earlier, the mucous layer 

 has become more irregular and assumed a streaked appearance, 

 due to the distinctness of the cell walls. 



By the seventeenth day (fig. 23) other changes have occurred. 

 No more layers exist, but the horn layer is much thicker and the 

 horn cells have become well developed. That is, they have 

 assumed a roughish surface, rather shrunk in size, and have lost 

 their active albumen. Most reagents will not stain them, and 

 picrocarmine gives the yellow color peculiar to horn. When 

 stained with a solution of eosin they also assume a peculiar 

 yellow color. 



A little later other changes occur, which bring the embryonic 

 development of the claw to a close. The granular layer begins 

 to dry up and becomes fibrous, thus loosening the epitrichial 

 layer. These two layers are shed at or about the time of hatch- 

 ing and leave us the adult claw. 



In no case, even in black chickens where the scales were full of 

 pigment granules, have I observed pigment in the claws; yet 

 there is no reason, so far as I know, why it should not occur 

 there. 



The claws on the wing develop very much later than those on 

 the foot. The horn does not seem to form until near the time of 

 hatching. I have sections from both ducks and hens, cut on the 

 sixteenth and seventeenth days, where no claw is formed. The 

 skin, however, in such cases is smooth and thicker than in other 

 parts. The time of development of the hand claws is variable, 

 as one would expect of aborted organs. It is always late, and 

 probably does not occur till after hatching in many species. 



To sum up, the layers of the claw are continuous with those of 

 the tarsus, and like them in structure, the only difference being 

 in their relative thickness. 



THE BILL. 



In reality the structure of the bill in adult birds is often com- 

 plex, owing to the development of sensory organs. But, for our 

 purposes, it may be regarded as being built just like the claws, 



