374 



Such is the coDstitution of the adult claw, as determined by Boas, 

 who did not work at their development. I find, however, that in the 

 embryos of all the classes of unguiculate vertebrates there is present 

 an additional structure, which, as it is only present in embryonic life, 

 I propose to call the neonychium. I will first describe the diflferen- 

 tiation of the main parts of the claw in the armadillo Tolypeutes, in 

 which the neonychia are better developed than in any other form 

 I have met with. I have purposely abstained from going into details 

 of histogenesis. 



The claws of the foetus of T. couurus^) presont a remarkably 

 ditferent appearance from those of the adult, their tips being much 

 expanded instead of ventrally curved and sharply pointed, and pre- 

 senting a certain superficial resemblance to the hoofs of the Ungulata. 

 Closer examination showed however that the hoof-like appearance of 

 the claws is entirely due to the fact that the points are enclosed in 

 soft pads which are purely provisional foetal structures. These pads 

 are what I have spoken of as the neonychia. 



The condition of the claws of Tolypeutes at diöerent stages is 

 shown by the accompanying figures. Fig. 1 represents the left manus, 

 seen from the outer aspect of a foetus 12.5 cm in length, excluding 



Neonychia 



Fig. 1. Fig. 2. 



Fig. 1. Left manns of a 12.5 cm foetus of T. conurus. 

 Fig. 2. Left manus of an adult. 



the tail. The neonychium is seen on all the claws. In Fig. 2 is 

 shown the left manus of an adult for comparison. Figs. 3 and 4 are 

 median longitudinal sections of claws of two foetuses of ditferent ages. 

 Fig. 3 is taken from a 9 cm foetus; the other one. Fig. 4, is from a 

 specimen of the same age as Fig. 1. 



1) These foetuses were collected during a zoological expedition to 

 the Paraguayan Chaco. The expenses of this expedition were defrayed 

 partly by the Government Grant Committee of the Royal Society and 

 partly by the Managers of the Balfour Fund at Cambridge University. 



