520 EMBRYOLOGY. 



III. The Development of the Skin and its Accessory Organs. 



Having now become acquainted with the physiologically more 

 important functions of the outer germ-layer, which consist in the 

 production of the nervous system and the sensory organs, I give a 

 short survey of the changes which take place in the remaining part, 

 which is now designated as primitive epidermis (Hornblatt). This 

 furnishes the whole outer skin of the body or epidermis and the 

 numerous and various organs that are differentiated out of it, such 

 as the nails, the hair, and the sweatr, sebaceous, and milk-glands. 



(a) The Skin. 



The epidermis of Man is, according to the statements of KOLLIKER, 

 very thin during the first two months of development, and consists of 

 only two single layers of epithelial cells. Of these the superficial 

 layer exhibits flattened, transparent, hexagonal elements ; the deeper 

 one, on the contrary, consists of smaller cells ; so that already there 

 is indicated by this a differentiation into a corneous and a mucous 

 layer. Even now, too, a detachment of epidermal cells begins to 

 manifest itself. For the outer cell -layer is soon found to be in 

 process of decay, with obliterated cell -contours and indistinct nuclei, 

 while a supplementary layer arises beneath it'. In many Mammals 

 the dying layer of cells is detached as a continuous sheet and 

 then constitutes for a time a kind of envelope around the whole 

 embryo, to which WELCKER has given the name epitrichium, because 

 the outgrowing hairs are developed beneath it. 



From the middle of embryonic life onward both layers of the 

 epidermis become thicker and the outermost of them contains 

 cornified scales, the nuclei of which have degenerated. From this 

 time onward a more extensive desquamation takes place at the 

 surface, while the loss is made good by cell -divisions in the mucous 

 layer and by the metamorphosis of these products of division into 

 cornified cells. In consequence of this the surface of the embryo 

 becomes up to the time of birth more and more covered with a 

 yellowish-white, greasy mass the smegma embryonum or vernix 

 caseosa. This consists of a mixture of detached epidermal scales and 

 of sebaceous secretions, which have been produced by the dermal 

 glands that have arisen meantime. It forms a thick layer, especially 

 on the flexor-side of the joints, on the sole of the foot, the palm of 

 the hand, and on the head. Detached portions of it get into the 



