412 



THE SKIN. 



contained principally in the cells of the deep layer or the rete mucosum (fig. 468), 

 but even the superficial part possesses a certain degree of colour. 



In the intercellular channels of the Malpighian layer lymph-corpuscles, rounded 

 or branched, are occasionally observed, and these, in the negro, may also contain 

 pigment-granules. It has been supposed that these cells may act as carriers of 

 pigment to the epidermis, since leucocytes containing pigment are also found in the 

 negro in the superficial layers of the cutis vera. This is also the case in the bronzed 

 skin of Addison's disease. 



Development of the Epidermis. The epidermis is derived from the cuta- 

 neous epiblast. Up to about the second month of intra-uterine life it consists, 

 according to Kolliker, of two layers of protoplasmic cells, of which the deeper are 

 smaller and rounded, the more superficial larger and polygonal. The latter multiply, 

 and in the second month are two or three cells deep ; they are clearer and more 

 flattened out than the deep layer of cells, and stain less deeply with carmine. 



Fig. 469. SECTION OP EPIDERMIS FROM THE OCCI- 

 PUT OF A FCETUS OP 2J MONTHS. (Bowen. ) 



Fig. 470. SECTION OP EPIDERMIS FROM THE PALM 



OF A 3 MONTHS FCETUS. (BoWCU.) 



ep, epitrichial layer. 



Between the second and third month there 

 is found, according to the observations of 

 Minot and Bowen, forming the most super- 

 ficial layer of the epidermis, a stratum of 

 enlarged, swollen-out cells (fig. 469). 

 These were first noticed by Zander upon 

 the fingers and toes of embryoes of about 



thirteen weeks or more, and were termed by him " bladder " cells. The layer 

 increases for a time in thickness, so as to be two or three cells deep (fig. 470), 

 or even in certain parts, e.g., over the situation of the developing nails and on the 

 palmar surface of the hand and feet, to acquire a considerable thickness. Here the 

 stratum seems to persist, but it disappears over the body of the nail, and also 

 over the remainder of the surface of the skin, so that by the sixth month it is 

 only found near the free border and root of the nail, and perhaps forming the thick 

 horny layer of the palms and soles, which belongs to the second type of horny layer of 

 Zander. This layer of bladder-like cells appears to correspond with an epithelial 

 membrane which was first noticed by Welcker in a sloth-embryo, covering the 

 surface of the body and lying over the developing hairs, hence named by him 

 epitrichium. It has accordingly been termed the epitrichial layer, and has been 

 shown to be of wide occurrence in the embryoes of mammals and birds, and also to 

 be represented in reptiles. 



The cells of the foetal epidermis underneath this epitrichial layer form the 

 Malpighian layer, and eventually the more superficial develop granules of eleidin and 

 become keratinized, so that a stratum corneum is produced. After the throwing 

 off of the epitrichial layer, the superficial cells of the stratum corneum also become 

 gradually cast off, whilst others pass from the Malpighian layer to replace them. 

 The cast-off scales, mingled with secretion of the cutaneous glands, form it with a 

 yellowish caseous layer covering the surface of the foetus (smegma embryonum, 

 vernix caseosa), and occurring also in flakes in the amniotic fluid. 



The pigmentation of the Malpighian layer in the coloured races of mankind 

 frequently does not appear until a day or two after birth ; probably because it is 

 concealed by the moist and therefore opaque epidermis over it. In the negro the 

 pigment is certainly present even some weeks before term. 



