444 



them the smaller cells of the rete Malpighii. In the later periods 

 many of the bladder cells are shrunken and contracted, when the cell 

 boundaries are often lost. 



These outer epidermal cells, at first a single layer of large 

 polygonal cells, afterward increasing in numbers and assuming bladder, 

 domed, and irregular shapes; characterized by their large nuclei, 

 granular appearance and strong reaction to staining agents, I propose 

 to call the epitrichial layer. Undoubtedly Welcker meant these cells 

 when he described a human epitrichial layer. He described in an 

 embryo of 8 weeks an outer layer of large, polygonal cells 0,020 mm 

 — 0,035 mm long; 0,015—0,025 ram broad and under them the cells 

 of the future epidermis only 0,007 — 0,010 mm in size. Further in an 

 embryo of 4 months, the outer epidermal cells 0,020 — 0,036 in length, 

 0,020 — 0,028 in midth. At 8 months the epitrichial layer had dis- 

 appeared. 



It seems that there are good and sufficient reasons for cons'dering 

 these cells as forming a separate layer of the epidermis. 1. They 

 are larger than the cells below, contain a larger nucleus, are more 

 deeply stained, and present a granular appearance. 2. They are 

 sharply bounded from the cells below, none of which ever attain the 

 large size of the upper layer. 3. They assume peculiar, swollen, 

 bladder forms, which give the appearance of a degeneration of this 

 layer. The cells of the mucous layer never assume these appearances. 

 4. They are concerned in the formation of the stratum corneum, 

 only, as will be explained below, on certain parts of the body. On 

 most portions the horny layer is produced solely from the mucous 

 layer and the epitrichial layer is not concerned in its production. By 

 the time the stratum corneum of most parts of the body has appeared, 

 the bladder cells have either disappeared entirely, or, as I have occa- 

 sionally seen, remain at wide intervals clinging to the surface scales. 

 These cells moreover correspond in general with the cells forming the 

 epitrichium, or membrane covering the hair, found in certain animals, 

 which are, like these cells, large elements with a large nucleus, when 

 contrasted with the smaller cells lying below. (Welcker, loc. cit. 

 Tafel II, Fig. 2 a, h.) 



The similarity of these cells to those forming the epitrichial 

 layer of certain animals was seen, in the case of the chick, by spe- 

 cimens kindly lent me by Dr. Jeffries. A number of specimens from 

 the toe of the chick showed an upper layer of large, granular, polygo- 

 nal cells, below which was a horny layer consisting of continuous scales. 

 Surface views from the same part showed the outer layer of large 



