Tefiries.] 224 [February 7, 



The shape of the claw varies according to the kind of Bird and 

 some have claws with a serrated edge. Whatever their shape 

 may be, they are composed of horn-cells produced by the mucous 

 cells covering the ultimate phalanx of a digit. The base of the 

 claw is sunk in a socket or overlapping fold of skin. The growth 

 of the claws is not directly outwards from the mucous layer, but 

 outwards and tipwards, so that the claw is constantly sliding off 

 the end of the digits just the same as our finger nails slide off the 

 ends of our fingers. 



There is very little to say about the histology of the claws ; 

 it is much the same as in the mammals. The mucous cells are 

 columnar and separated by walls as in the scutae. Outside the 

 mucous layer is a thick bed of transitional cells much like those 

 of the scutae. The outer ones, however, show in texture a grad- 

 ual change to the horn-cells. So when sections of the claw are 

 stained with picrocarmine there is no sharp line between the red 

 and the yellow, but the red slowly fades out towards the surface. 

 The horn-cells form a thick layer, being thickest in the large 

 scratching birds and thinnest in the small arboreal species. 

 Beneath the mucous layer is the cutis vera, and below this the 

 bone. The outer part of the cutis vera is very vascular and 

 large capillaries may be seen running just below the mucous 

 layer. 



How the combs on the claws are formed I cannot say. Accord- 

 ing to Macgillivray (20) they are absent in the young bird, and 

 are later formed by the splitting of an even lamella. This may be 

 the correct solution of the problem, but more data must be got 

 before it is accepted. It seems probable that the gaps are formed 

 by some of the cells being softer than others and falling out, as 

 we shall later find to be the case in the tongue. 



I know of no data pointing to a moult of the claws, though 

 such a moult may exist. The claws would seem simply to grow 

 and wear off like our nails. 



DEVELOPMENT OF CLAWS. 



Nothing, so far as I know, has been written on the embryonic 

 growth of bird's claws. Their development is of interest since 

 there can be no doubt of the morphological identity of claws 



