413 



and the cells themselves to be part of a superficial layer, homologous 

 with the so-called epitrichiura that has been described in birds and 

 reptiles ^). 



It will be my endeavor to prove that there exists in the human 

 epidermis, during a certain period of foetal life, an outer layer of 

 cells that has heretofore received but scant notice, and that this layer 

 probably finds its prototype in the epitrichium of certain animals. 



The first mention of an epitrichium is found in an article published 

 by Welcker, professor in Halle ^). Welcker found in an embryo 

 of Bradypus (sloth) a peculiar membranous covering, beneath which 

 appeared the thick hair development of the animal. This proved on 

 investigation, to be the outermost layer of the foetal epidermis that 

 was at first continuous with and a part of the epidermal layers im- 

 mediately above the rete, and had been raised up and separated from 

 the rest of the skin by the development of the hairs. Mention had 

 previously been made of this covering by Eschricht and Ebsen^), 

 who regarded it as a part of the amnion, and by Simon *), who was 

 inclined to consider it a skin peculiar to the foetus, and not a part 

 of the epidermis. To this external membrane Welcker gave the 

 name epitrichium , as descriptive of its position , in that it formed a 

 complete covering of the hair. Investigation of other embryos con- 

 vinced Welcker that a similar covering exists in a certain number 

 of mammals, while in others, although no distinctly separable mem- 

 brane occurs, the upper cells of the ectoderm form a layer histologic- 

 ally difi'erent from the underlying cells. To these cells Welcker 

 gives the name epitrichial layer, as homologous with the epi- 

 trichium, which term, as has been stated, he reserves for the separable 

 membranous covering above the hairs, found only in certain mammals. 



Coming lastly to man, Welcker describes an epitrichial layer 

 consisting of large polygonal cells with round nuclei, much larger than 

 those of the underlying epidermal layers. According to this author 

 an epitrichium (meaning thereby a freely separable membrane, covering 

 the hairy growth) occurs in Bradypus, Choloepus, Myrmecophaga, 

 Dicotyles, Sus, and probably in the horse. 



Altough Welcker's epitrichium has been admitted pretty generally, 

 his epitrichial layer has not been universally accepted, at least with 



1) American Naturalist Extra, June 1886. 



2) Über die Entwicklung und den Bau der Haut und der Haare bei 

 Bradypus, Halle 1864. 



3) Müllee's Archiv, 1837, S. 41. 



4) Mülleb's Archiv, 1841, S. 370—372. 



