DEVELOPMENT OF THE SKIN. 485 



those underneath, the stratum Malpighii. Then, as the former be- 

 comes detached, it is recruited from the latter. The extension in 

 surface of the cuticle implies a series of desquamations in the em- 

 bryo and the fcetus, and which must also occur after birth. The 

 multiplication of the cells in the stratum Malpighii is certainly not 

 by free cell-development (p. 120) ; since at no age are free nuclei 

 present in it. (Kolliker.) In the embryo of five weeks, there are 

 but two layers of cells instead of the epithelium ; at fifteen weeks, 

 three layers of cells, the two internal for the stratum Malpighii ; in 

 the fifth month, the latter consists of many layers of the smaller 

 cells, and the cuticle of at least two, of polygonal flattened cells ; 

 and at the seventh month, these two layers are as sharply distin- 

 guished from each other as in adults. In the new-born infant the 

 epithelium resembles that of the adult, except that it is more easily 

 separated from the corium by maceration, and that the stratum 

 Malpighii is disproportionally thick, and the cuticle very delicate. 



The desquamations of cuticle during embryonic life, already 

 alluded to, aid in the formation of the vernix caseosa, already de 

 scribed (p. 226) ; this consisting of the external epidermic cells 

 mixed up with the sebaceous secretion of the skin, and containing 

 hairs ; and which, especially from the sixth month onwards, covers 

 the whole surface of the fcetus. It varies greatly in quantity in the 

 new-born child ; sometimes amounting to even 8J drachms. 



The pigment in the stratum Malpighii of the negro, appears after 

 birth. The edges of the nails, and the surface around the nipple, 

 become rapidly tinged black ; the genital organs become colored on 

 the third day, and the whole body on the fifth and sixth. (Camper.) 

 In Europeans also, the pigment in the areola is gradually developed 

 during the first year. 



The growth of the corium presents no peculiarities. The cuticle 

 is coostantly being detached and repaired, and is thus constantly 

 growing. The cells of the stratum Malpighii are developed from 

 plasma exuded from the bloodvessels of the corium ; and of which 

 a determinate quantity always exists among these cells, and even 

 those of the cuticle also. In the deep fold of the skin surrounding 

 the glans penis and clitoridis, this continuous desquamation and re- 

 production of the cells of the cuticle produce the substance (not 

 a secretion, as generally supposed) called the smegma prceputii. In 

 the male, however, the secretion of Tyson's glands may be mixed 

 with it; but in the female, neither sebaceous nor any other glands 



