444 



Mr. W. E. Clarke on the 



tiuized. There was not, however, a trace of a nail to be 

 found on the hallux, or even a thickening of the epidermis 

 in the nail region, in either series of transverse and longitu- 

 dinal sections prepared from this specimen. 



The next specimen was a ripe embryo estimated to be 

 twenty-five days old. Before sections were prepared the 

 hallux of this specimen was examined under a magnifying- 

 power of 50 diameters, which showed this toe to be a mere 

 papilla, clothed in scutellate epidermis, and furnished with a 

 nail placed almost centrally upon the dorsal aspect, being at 

 some little distance fi'om the apex of the digit and from its 

 lateral margins. This nail was small in size and oblong- 

 oval, like a patella in shape (see fig. 1), its longest diameter 



Fig. 1. 



Dorsal aspect of hallux of ripe embryo of Rissa tridactyla^ sliowing the 

 position of the nail. X 50. 



E, edge of nail ; 0, centrum of nail ; A., apex of hallux. 



corresponding with that of the toe. The sections cut show 

 that the nail is embedded at its edge under a circular nail- 

 fold, towards which the nail gradually tapers down from its 

 somewhat raised, thickened, central portion. In structure 

 it resembles an ordinary nail, and tends to divide into lamellae, 

 or '' flake/' when cut in a microtome. In position the nail 



