415 



that appears before the embryo is sufficiently developed to produce 

 a true horoy layer. He lays stress on the fact that at an early period 

 of development (his studies were directed to the chick and lower ani- 

 mals) it is impossible to say whether the cells produced from the 

 mucous layer are converted into horny cells, or whether they remain 

 without horny change and cover the stratum corneum. He showed 

 that in the embryos of the chick and of mammals, there is developed 

 on those parts that form a true horn, a layer of specific histological 

 appearance, which is situated above the stratum corneum. In the 

 chick he finds no epitrichial layer on those parts in which no real 

 horn substance is formed, and says that on the back, head, etc., this 

 layer cannot be distinguished from the underlying cells. The outer- 

 most epidermal cells correspond to those of the adult animal, in that 

 they are flattened and not so vigorous. Gardiner further studied 

 the epitrichium of the pig's hoof. In the latter he found a distinct 

 layer of round or oval cells lying over the horn of the hoof, well 

 shown in his fig. 16. 



Unna ^) has seen the epitrichium of the pig's embryo unite with 

 the hoof. He considers that in man there exists an epitrichium only 

 on the nail which would be connected with an epitrichium of the 

 entire body, if the horny layer possessed a greater firmness. This 

 horny layer covering the nail he names eponychium, following Welcker's 

 name, and because the later nail really appears under this peculiar 

 covering. To this eponychium reference will again be made. 



In undertaking these investigations, Dr. Minot's collection of 

 embryos, numbering several hundred, was placed at my disposal. The 

 outer epidermal layers proved to be very easily detached in the pro- 

 cess of imbedding and cutting, and many young embryos which were 

 apparently well preserved were found, after the sections were mounted, 

 to be useless for this work. Some of the embryos were hardened in 

 alcohol, some in MIjller's fluid. Very great assistance was obtained 

 by using the new microtome of Dr. Minot ^) for serial sections. The 

 methods which proved most satisfactory were staining in toto in picro- 

 carmine or in alum cochineal and eosine, then imbedding in paraffine 

 and cutting with the above mentioned microtome, when sections could 

 be obtained ^/aoo ^^ i" thickness. Numerous pieces were also im- 

 bedded in celloidine and stained in various ways. 



1) Ziemssen's Handbuch der Hautkrankheiten, I, S. 39. 



2) Zeitschrift f. mikroskop. Anat., Bd. V, Heft 4, 1888, S. 474. 



