414 



regard to man, and his observations have been so far forgotten that 

 no mention of these cells is made in most of the text books in their 

 chapter on the development of the human skin ; although there can 

 be no question of their presence whatever may be their significance. 



Kerbert^), whose investigations were chiefly directed to the 

 skin of reptiles, showed that the outermost epidermal layer was not, 

 as Leydig had supposed, a cuticula (i. e. a part secreted from the 

 horny layer), but a distinct histological layer, which he named epi- 

 trichial layer, following Welcker's observation. This is described as 

 a layer of polygonal cells with occasional nuclei, while in the layer 

 below the cells are round, with a larger nucleus. Kerbert also de- 

 scribes in reptiles a granular layer (Köruerschicht) immediately below 

 the epitrichial layer, and lying directly upon the stratum corneum, 

 which he considers as separate from the epitrichial layer. 



KöLLiKER ^), to whom we refer for so much of our knowledge of 

 the early stages of the epidermal development, remarks upon the dis- 

 covery of an epitrichium by Welcker in mammals, but considers that 

 in man we have no ground for separating the primitive horny layer 

 from the later horny layer. He says it seems to be thus: that the 

 embryonic horny layer that is first developed, is sooner or later cast 

 off. On the other hand he says it is not proved that everywhere 

 and in the first instance it is only the outermost layer that is cast 

 off, and that there is a distinct difference between this and the next 

 following layer. 



Jeffries^) in studying the epidermal system of birds, found in 

 embryos of the 5*'' day an epitrichial layer distinctly formed; on the 

 20"" day it had „assumed its final form which is the same all over 

 the body. It now consists of a continuous layer of flat polygonal 

 cells, with broad divisions between them, of about 0,015 mm in width 

 and one fourth of that thickness". These epitrichial cells he con- 

 siders undergo division and proliferation. The epitrichial layer and 

 some of the underlying cells, dry up and are shed after hatching. 



Gardiner ^) considers the epitrichial layer a part of the epidermis 



1) CoENEAAD Kekbeet, Über die Haut der Reptilien und anderer 

 Wirbeltiere, Archiv f. mikrosk. Anat., Bd. XIII. 



2) Entwicklungsgeschichte des Menschen, Leipzig 1879. 



3) J. A. Jefpeies, The epidermal system of birds. From the Pro- 

 ceedings of the Boston Society for Natural History, Yol. XXII, February 7, 

 1883. 



4) E. G. Gaedineb, Beiträge zur Kenntnis des Epltrichiums und der 

 Bildung des Vogelschnabels, Leipzig 1884. 



