Jeffries.] 228 [February 7, 



the body of the cell. Each nucleus is provided with a fair- 

 sized, very dark, highly refractive nucleolus. The whole mass of 

 horn cells forms a layer about twelve cells deep in the thicker 

 parts, gradually shading off to nothing on the edges. Beneath 

 the horn cells are a few transitional cells ; their small number 

 being presumably due to their rapid transition into horn cells. 

 Lastly comes the mucous layer, composed of the usual cells and 

 the same irregular basal border that has been described in the 

 advanced claw. In fact sections at this time of the mandible and 

 claw are very much alike structurally. 



By the thirteenth to the fourteenth day there are further 

 changes ; I refer to the formation of the egg tooth. This is, in re- 

 ality, nothing but a sort of conical prominence of horn cells, de- 

 veloped over the body of the premaxillary bones. It is formed by 

 the more rapid growth of the horn cells below, and by the greater 

 solidity attained by them. By the fourteenth day they are 

 unstainable with the ordinary reagents, though a benzole eosin 

 mixture will stain an occasional patch of protoplasm or nucleus, 

 and become bright yellow by the action of picric acid. So their 

 activity has ceased. In certain individuals at least, zones of pig- 

 ment granules can be seen in the egg tooth. 



Below the hardened horn cells comes a bed of horn cells like 

 those described in the six-day chick. Yet again below the horn- 

 cells are the transitional cells and the mucous layer, in certain 

 cases charged with pigment. From this time until after hatching 

 there are no changes worthy of special description. There is 

 simply a gradual growth of the horn cells, as a consequence of 

 which the egg tooth is slowly pushed through the granular and 

 epitrichial layers. At or about birth the epitrichial, granular and 

 ill-formed horn cells are shed. Thus we see that the growth 

 of the bill in the chick is much the same as that of the claws ; 

 the difference being the presence of the egg tooth. In the lower 

 mandible there is really no difference. 



COMBS AND WATTLES. 



The combs and wattles are simply flaps of the true skin, 

 covered by the epiderm. The epiderm on them is simple, com- 

 posed of mucous, transitional and flat horn cells, and usually thin 

 in order to allow the blood to be seen. The cutis vera is very 



