162 MANUAL OF HISTOLOGY. 



were smaller, about '0068-0 -0090 mm. in diameter, with nuclei of only 

 '003 40' 0045 mm. From this we see that the epidermis, properly so 

 called, and the rete Malpighii, are each formed originally of one layer 

 of cells. Later on in the fourth month these are slightly laminated, 

 the whole epidermis consisting of three or four strata (fig. 152, a b). 

 From this on the lamination becomes stronger by degrees. Let us take, 

 for example, the epidermis of a foetal sheep 4 inches long (fig. 153). 

 This consists of six or seven layers of cells (fig. 153), of which the 

 most superior (a) are transparent, and measure 0'0156-0'0206 mm., with 

 nuclei 0'0052-0'0066 mm., whilst the deeper (b) are only 0'0104-0'0124, 

 the nuclei preserving the same dimensions as in the more superficial 

 laminre. In the superficial strata are found scattered cells with a double 

 nucleus (fig. 153, 1), and division of the latter may be remarked at times 

 in those lying deeper (2). The epithelium on the free edge of the eye- 

 lid shows in this embryo but two layers of cells (4). I have found the 

 epithelium of the cornea also in a human foetus four months old 0'0205 

 mm. thick, and consisting of two upper and two lower layers of 

 cells. 



With the further growth of the foetal body, the thickness of the epi- 

 dermis and the number of its laminae increase more and more, and the 

 most superficial of the latter already resemble the scaly non-nucleated 

 structures of later years, before the close of the second half of intra- 

 uterine life. 



Desquamation of the cuticle commencing in embryonic life, produces 

 upon the body of the child a coating of a greasy whitish substance, 

 intermingled with fat, known as the vernix caseosa, in which the micro- 

 scope reveals to us the scale-like epidermal cells. 



The epithelia, also, of the intestinal glandular plate likewise evince at 

 an early period a disposition to assume their characteristic forms. The 

 increase of the superficial extent of these coverings necessitates likewise 

 multiplication of the cells by division. 



That the endothelia also make an early appearance has been already 

 observed above, where we have also considered the cells from which they 

 take their rise. 



REMARKS. 1. Conip. the work on embryology of this investigator. 2. In a re- 

 cent work His points out that from a superior germinal plate the nervous system, 

 animal muscles, Wolflian bodies (the kidneys and sexual glands), take their rise 

 together with the epidermal structures and cells of external glands. A subse- 

 quently formed inferior plate gives origin to the sympathetic system, unstriped 

 muscles, epithelia, and glands of the mucous membranes. These two plates con- 

 stitute liis " archiblast. " Between them the "parablast " is then inserted, from 

 which connective tissue and blood are formed. For the present we prefer adhering 

 to Remak's views. 



4. Nail. 

 99. 



Like the epidermis and the hair, to be considered farther on, the nails 

 have long been placed by anatomists among the horny tissues. And, 

 indeed, they represent nothing more than a peculiarly modified cuticle 

 for the part of the skin lying underneath. But the transformation 

 proves to be less on microscopic investigation than we might have 

 expected from the physical constitution of the tissue. 



The nail is a hard, flat and arched body of rounded quadrangular 



