OF THE SKIN. 155 



In an embryo of five weeks I found, in the place of the epidermis, 

 nothing but an external layer of polygonal cells of 0-012-0-02 of a line 

 in diameter, and an internal layer of small cells of 0-003-0-004 of a line ; 

 in embryos of fifteen weeks the epidermis is 0-01-0-012, of a line thick, 

 and composed as before, only that the deep layer of cells answering to 

 the stratum mucosum is often already double, and the external cells 

 measure only 0-009-0-012 of a line. 



In the fifth month the epidermis in one instance measured on the heel 

 and ball of the hand 0-02-0-024 of a line, over the ridges of the cutis 

 0-036-0-04, in the furrows between them on the back 0-02-0-024 of a line, 

 of which thickness one-third may be regarded as belonging to the horny 

 layer and two-thirds to the rete Malpigliii. In a somewhat older embryo 

 it was, on the heel 0-06--064 of a line (mucous layer 0-05, horny layer 

 0-01-0-014), on the surface of the hand 0-05 of a line (mucous layer 0-04, 

 horny layer 0*01), on the back 0-02-0-024 of a line (mucous and horny 

 layers of equal thickness). The mucous layers consisted of many layers 

 of smaller cells, the lowest of which were already elongated, and stood 

 perpendicularly ; the horny layer, of at least two layers of polygonal 

 flattened cells, with round nuclei. 



In the sixth month the epidermis upon the thorax measures 0*02 

 0-022, on the palm of the hand 0-06, on the sole of the foot 0*07 of a 

 line, and everywhere it consists of many layers of cells. The outermost 

 one or two are composed of horny plates without nuclei 0-01-0-14 of a 

 line, perfectly similar to those of the external horny layer in adults ; 

 then follow 3-4 layers of polygonal cells, the largest 0-01-0-012 of a 

 line, with nuclei 0-004 of a line; finally a mucous layer, whose thickness 

 equals about one-half or two-fifths that of the whole epidermis, with at 

 least 3 or 4 layers of rounded cells of 0-003-0-004 of a line, the lowest 

 of which are somewhat elongated, and are seated perpendicularly upon 

 the cutis. 



In the seventh month, I found in one embryo, that the epidermis 

 on the heel measured 0-12 of a line (mucous layer 0-072, horny layer 

 0-048), upon the back 0-07 of a line, (mucous layer 0-04 horny layer 

 0*03): in another it measured on the heel 0-120-14 of a line (mucous 

 layer 0-05-0-06, horny layer 0-07-0-08), on the knee 0-046-0-064 of 

 a line (mucous layer 0-016-0-024, horny layer 0-03-0-04). Both layers 

 of the epidermis are as sharply separated from one another as in adults, 

 and their elements similar to those of the perfect epidermis, especially 

 the lowest parts of the stratum Malpighii and the plates of the horny 

 layer, which have no nuclei, and measure 0-010*014 of a line in the 

 uppermost layers. 



In the new-born infant, apart from the thickness of the epidermis, 

 which in one case measured on the heel 0-10-11 of a line (mucous 

 layer 0-04-0-05, horny layer 0-06), nothing particular is to be observed, 



