THE INTEGUMENT AND THE EXOSKELETON 77 



sociated with the epidermis, while very loosely attached on 

 its under side to the parts which it covers, the two form to- 

 gether an easily detachable part, known as the skin or hide, 

 similar in general function to the integument of invertebrates, 

 but far more complex in structure. The vertebrate integument 

 is further characterized by a great variety of secondary struc- 

 tures, involving one or both layers and either remaining be- 

 neath the surface, as is the case with glands and pigment, or 

 projecting conspicuously beyond, as in hairs, feathers and 

 scales. 



Concerning the integument itself, in so far as it can be 

 treated apart from its accessory organs, it may be noted that 

 the epidermis is always several cells deep and is in constant 

 growth, being renewed from the innermost layer in about the 

 same proportion as it is worn off at the surface. This inner 

 layer is a fairly definite one and is termed the stratum gcrm- 

 inativiim [sir. mucosum or Malpighii]. Its cells are con- 

 stantly proliferating and the older cell generations are grad- 

 ually pushed toward the surface, becoming flattened and more 

 cornified as they progress. They thus form a protective cov- 

 ering for the more delicate cells that lie beneath them, and 

 compose a layer, which, in distinction to the stratum germ- 

 inativum, is called stratum cornenm. Some authorities dis- 

 tinguish for convenience a stratum lucidum, lying between the 

 two, although the exact limits of none except the stratum 

 germinativum are definitely fixed. 



It is evident that, in order to avoid an excessive growth of 

 these upper layers, there must be some way by which they 

 may be continually removed. This is accomplished in reptiles 

 and amphibians by periodic moults or ecdyses, through which 

 the entire surface layer is cast off by a single process, and 

 quite often in one continuous piece, after the formation of a 

 new layer beneath it. In many forms with a cornified skin, like 

 snakes and lizards, these cast-off " skins," the exuviat, are 

 matters of common observation, and are seen to reproduce 

 most faithfully every scale, horn or other protuberance charac- 

 teristic of the animal ; in certain other cases the cast-off skin 



