666 



COMPARATIVE ANATOMY 



layers, the lower one being known as the Malpighian layer or stratum 

 germinativum (Fig. 393). Usually this layer rests on the corium and 

 is nourished by the fluids from such corium. The cells therefore grow 

 outward as they divide, forming a second or outer layer, the stratum 

 corneum. These outer cells being the ones which come in contact with 

 the surrouading media, are worn away almost as fast as new ones are 



1 J>c ft on) 



Fig. 393. 



Diagram of a section through the skin of a mammal to show various layers, 

 hair, and sebaceous and sweat glands. 



added from below. If these outer cells come off in large sheets we find 

 such a condition as that of a snake shedding its skin. 



In land animals the first layer of cells budding off from the Mal- 

 pighian stratum seems a continuous sheet which is likely to be shed as 

 a whole. This is called the periderm (Fig. 395). Older books call this 

 the epitrichiurru but as this word means "above the hair" it is not accu- 

 rate when it refers to reptiles and birds which have no hair. 



Malpighian layer is that in and from which the glands of the 



