NORMAL HISTOLOGY. 



relation between the horny layer and the embryonal cells is still un- 

 settled. It is probable that where a well-developed stratum corneum 

 exists the parts of this external to the stratum lucidum represent the 

 metamorphosed epitrichium ; where, however, a true cornified layer 

 is wanting and the superficial cells belong to a highly-developed 

 stratum lucidum, as in the nails, the epitrichium is not represented, 

 since in this case the entire epidermis is derived from the deeper 

 layer of ectodermic tissue (Bowen, Minot). With the general 

 growth the layers of the epidermis increase in number and the 

 innermost cells assume the characteristic columnar character 

 which continues distinctive of the active Malpighian layer. 



The corium is formed of mesodermic tissue which becomes 

 condensed beneath the epithelial layer and subsequently is beset 

 with numerous papillary elevations ; the development of vascular 

 structures within the young corium takes place along with the dif- 

 ferentiation of a distinct subepidermal zone within the mesoderm. 

 Before the fourth month of foetal life the corium and the subcu- 

 taneous zones have become defined, and a little later fat-cells 

 appear within the last-named layer. 



The development of the nails depends upon the specialization 

 of the stratum lucidum within certain areas connected with the ter- 

 minal phalanges. The earliest indication of the nail-formation appears 

 about the third month in the human embryo, and consists of a thick- 

 ening of the primitive epidermis over the end of the digit ; the 

 nail-area becomes defined by a furrow and takes up a permanent 

 position on the dorsal aspect of the finger, when an ingrowth of 

 the stratum Malpighii takes place to establish the root of the 

 nail. About the fourth month the upper cells of the Malpighian 

 layer exhibit granules, which play an important part in the cornifi- 

 cation of the epithelial elements in the formation of the nail ; these 

 granules are very similar to, if indeed not identical with, eleidin in 

 their nature. The cells of the stratum lucidum subsequently un- 

 dergo great increase and constitute the body of the nail. Until about 

 the fifth month the young nail is covered superficially by the epi- 

 trichium, here called the eponychium ; the latter then disappears, and 

 finally is represented only by the small epithelial band, the perionyx, 

 which persists across the root of the nail. The final steps in the nail- 

 formation are associated with a process of desquamation of the 

 stratum lucidum, whereby the distal end of the nail is separated 

 from its bed and the existence of a free edge is established. 



By the addition of young cells at its posterior margin the nail 

 grows in length, while by the increments to its under surface 

 derived from the stratum mucosum at the lunula it increases in 

 thickness ; the thickest part of the nail is, therefore, not at its 



