1883.] 225 [Jeffries 



throughout the higher vertebrate groups. Hence they may be 

 taken as a basis from which we can recognize any layers of the 

 epiderm, which are of morphological value, rather than the acci- 

 dental results of the laws of growth. 



Claws seem to be marked out on the ends of the toes as soon as 

 these members are developed. By the sixth day the claws can 

 be perceived in form, but histological distinctions do not begin 

 till later. 



By the twelfth day a claw is clearly visible to the naked eye 

 It is simply the smooth skin covering an ungual phalanx which 

 is roughly claw-shaped. Longitudinal sections through the toe 

 and claw show that the epiderm is thicker than the proximal part 

 of the toe; the increase in thickness being due to the larger 

 number of developing horn cells. The whole is more advanced 

 than the rest of the toe ; layer for layer, however, the claw is just 

 like the rest of the toe skin. 



By the middle of the fourteenth day marked distinctions exist. 

 (Fig. 24.) The claws have a more solid appearance to the eye, 

 and sections show considerable differences. The epiderm is two 

 or three times as thick as on the rest of the toe. Here it must be 

 borne in mind that the thickness of the epiderm varies in the 

 young claw as in the adult ; so the end is thicker than the base, 

 The epitrichial layer is thinner, if anything, and the cells closely 

 joined; thus in section they seem to form a structureless mem- 

 brane, but in surface views the polygonal cells are clearly seen. 

 Beneath the epitrichial is the granular layer (fig. 24&). This is 

 built up of cells placed close together, almost fused into a mass, 

 with indistinct nuclei. In parts the granular layer is only one 

 cell thick, in others very much more. The whole has the aspect 

 of being made of cells which have begun to develop into horn 

 and then gradually lost their activity, and been squeezed up by 

 those below. Next follows the horn layer. This is built up of 

 fusiform cells, running lengthwise, provided with elongated 

 nuclei and dark nucleoli. The protoplasm at this period is dis- 

 tinct. The horn layer and the forming cells below are the chief 

 cause of the increased thickness of the epiderm in the claws. At 

 the base of the claws both layers are very thin, but thicken up 

 at the tip of the toe. Beneath the horn cells are the transitional 



PROCEEDINGS B. S. N. H. VOL. XXII. 15 DECEMBER, 1883. 



