GENERAL HISTOLOGY 



07 



changes of form, become the superficial layers, here to replace an equal 

 quantity of worn-out cells (fig. 27). 



In the course of this change of position, the cells may undergo an altera- 

 tion; in the reptiles, birds, and mammals (fig. 28) they became cornified, 

 first the margins, then the inner part of the cell, changing into horn 

 {^keratin 1 } . The nucleus remains for some time, but at length this vanishes, 



-Xo 



Co 



FIG. 27. Section through the skin of Petro- 

 myzon planeri. Ep, the many-layered epithelium 

 of the epidermis, including B, goblet cells; Kd, 

 granular cells; Ko, club-cells; Co, corium (with 

 blood-vessels, G), consisting of bundles of fibrils 

 running horizontally (W) and vertically (5) (from 

 Wiedersheim). 



FIG. 28. Stratified epithelium 

 of man. 5.17, stratum Malpighi; 

 sc, stratum corneum. 



FIG. 29. Single-layered epithe- 

 lium of a snail, c, cuticle; d, 

 goblet cells. 



and then the cell becomes completely changed into a dead, horny scale. 

 In the skin of the higher vertebrates the zones of the living protoplasmic, 

 and the cornified cells, are sharply marked off from one another. In 

 cross-section they are readily distinguished as the stratum corneum (sc) 

 and the stratum Malpighi (sM) of the skin (fig. 28). In the many- 

 layered epithelia the cuticle has lost its importance, and it is either an in- 

 conspicuous boundary line or is entirely absent. 



