526 EMBRYOLOGY. 



In addition to the development of hairs from old fundaments, a 

 second method of formation, -whidi one might designate as direct or 

 primary /.is maintained! by many writers (GOETTE, KOLLIKER). It is 

 assumed that even after birth, both in the case of Man and other 

 Mammals, hair-germs are formed diiei-tly from the mucous mem- 

 brane of the epidermis, in the same manner as in the embryo. In 

 how far, at what regions, and up to what age such a direct forma- 

 tion of hair takes place, demands still more detailed and exhaustive 

 investigation. 



(c) The Nails. 



A second organ resulting from a coriiification of the epidermis is 

 the nail, which corresponds in a comparative-anatomical way to 

 the claw- and hoof -like structures of other Mammals. In human 

 embryos only seven weeks old there appear proliferations of the 

 epidermis at the ends of the fingers, which are noticeably short and 

 thick, and likewise at the ends of the toes, which are always less 

 developed than the fingers. In consequence of the proliferations 

 there arise from the loose epidermal cells complicated claw-like 

 appendages, which have been described by HENSEN as predecessors of 

 the nails or primitive nails. 



In somewhat older t mbryos, from the ninth to the twelfth week, 

 ZANDER found the epidermal growth marked off from its surround- 

 ings by a ring-like depression. The growth consists of a single 

 layer of cylindrical cells with large nuclei lying on the side toward 

 the derma and corresponding to the reto Malpighii, of two or three 

 layers of polygonal spinous cells, and of a corneous layer. 



The territory thus distinguished by a depression and by an 

 altered condition of the cells ZANDER calls the primary basis of the 

 nail (Nagelgrund), and describes it as occupying a greater part of 

 the dorsal, but also a smaller part of the ventral surface of the 

 terminal segment. He infers from this that the nails in Man 

 originally had, like the claws of the lower Vertebrates, a terminal 

 position on the toes and fingers, and that they have secondarily 

 migrated on to the dorsal surface. Thus he explains the fact that the 

 region of the nail is supplied with the ventral nerves of the fingers. 



GEGENBAUR subscribes to ZANDER'S view of the terminal position 

 of the fundament of the nail, but, supported by the investigations 

 of BOAS, opposes ZANDER'S assumption of a migration of the funda- 

 ment of the nail dorsally. He distinguishes in the development of 

 nails and claws two parts (fig. 293), the dorsally located firm nail- 



