1883.] 227 [Jeffries. 



and in fact this is the structure in hard billed birds. Soft billed 

 birds have the mandibles covered with ordinary skin with many 

 sense papillae. Lamellae, such as those found in the Mergansers, 

 are produced by the more rapid growth of the horn cells in ridges, 

 and by folds of the epiderm. 



DEVELOPMENT OF THE BILL. 



In tracing the development I relied upon the chick as being 

 the bird best adapted to show the different layers as well as the 

 egg tooth to advantage. 



The time usually given by embryologists for the first appear- 

 ance of the horn cells is about the twelfth day ; I have seen both 

 the bill and the shell tooth distinct by the middle of the seventh 

 day. The first step towards the formation of the bill is the great 

 increase in number of transitional cells. These form a mound 

 on the upper surface of the mandible, which is separated by a 

 little groove from the tip of the bill, and by a groove with a flap 

 in it from the skin of the head. 



By the sixth day the epiderm of the mandibles is much more 

 developed than in any other part of the body. The epitrichial 

 layer has very nearly assumed its final appearance (fig. 25) and 

 the granular layer below has attained greater proportions than in 

 any other part, over the middle of the bill it is at least four cells 

 deep, shading off to nothing on the sides. It is made up of poly- 

 hedral cells packed close together, and connected by a transparent 

 intercellular substance. The cells themselves have fair sized 

 nuclei and are charged with granules about the size of the nucle- 

 olus. So in its structure and dimensions the granular layer of 

 the mandibles at six days strongly resembles the same layer as 

 developed on the claw at the tenth to the twelfth day. 



Below the granular layer comes the true horn layer. The outer 

 cells of this seem in patches to fail of development, and form a 

 sort of irregular transition between the granular layer and the 

 well developed horn cells. The latter are fusiform, their long 

 axes running parallel with that of the mandibles. The walls are 

 thick and tough, being composed of horny material, arranged in 

 such a way as to give, when stained, a peculiar appearance easily 

 recognisable but almost impossible to describe. The nuclei of 

 f he horn cells are of good size, slightly elongated, and placed in 



