THE ORGANS OF THE OUTER GERM-LAYER. 



527 



plate (np) and the plantar horn (Sohlenhorn, sh) connected witli- it 

 ventrally. Of these the latter arises from the smaller ventral 

 surface of the primary basis of the nail. In unguiciilate and 

 ungulate Vertebrates it (fig. 294 sh) is developed to a great extent; 

 in Man it atrophies, and is recognisable only in an exceedingly 

 reduced condition as nail-weU. By this term is meant the welt-like 

 thickening of the epidermis which forms the transition from the 

 bed of the nail to the corrugated skin of the ball of the finger. The 

 nail-plate, on the contrary, is from the beginning exclusively a 

 product of the dorsal surface of the basis of the nail. There is 

 therefore neither in Man nor in other Mammals a dorsal migration 

 of the terminal fundament of the nail, but only a degeneration of 



nw sh Tip 



B, Longitudinal section 



me wp sft 



Fig, 293. Fig. 294. 



Fig. 293. — A, Longitudinal section through the toe of a Gercopithecus 



through the second finger of Macacus ater. After Gegenbaor. 

 npt Nail-plate ; sh, plantar horn (Sohlenhorn) ; mo, nail-wall. 



Fig. 294. — Section through a Dog's toe. After Gegenbaur. 

 nii, Nail-plate ; sh, plantar horn : h, ball of toe. 



its ventral portion, which otherwise furnishes a more complete 

 plantar horn. 



So far as regards the particular events in the development of 

 the nail-plate, the structure is demonstrable in human embryos four 

 months old as a thin flat layer of cornified, closely united cells on 

 the dorsal surface of the primary basis of the nail or the bed of the 

 nail. It is produced by the mucous layer upon which it im- 

 mediately lies, but continues for a time to be covered by the thin 

 corneous layer that is present at all points of the epidermis. This 

 investment — Unna's eponychiimi — is not lost until the fifth month. 

 However, notwithstanding their investment, the nails are easily 

 recognisable some weeks before this from their whiteness, in dis- 

 tinction from the reddish or dark red color of the surrounding skin. 



