MULTICELLULAR ANIMALS 145 



a bony basal plate derived from mesodermal elements (Fig. 91). 

 The scales of common fishes are of mesodermal origin, the ectoderm 

 forming a thin layer covering the scaly armor. None of our living 

 Amphibia have scales, except as these are modified in the mouth to 

 form teeth. In many reptiles, however, the exoskeleton is well de- 

 veloped. The scaly nature of the integument of snakes, lizards, and 

 alligators is a matter of general information. The exoskeleton of the 

 turtle illustrates an extreme development of scales. The turtle armor 

 (Fig. 92) consists of two layers, a superficial layer of scutes derived 

 from the ectoderm and a nether layer of bony plates of mesodermal 

 origin. Beaks and claws are ectodermal and of separate origin. In 

 the bird the ectoderm is the layer of origin of the scales on the legs 

 and feet, of the beak and claws, and also of the feathers although a 

 mesodermal organ is concerned in the nutrition of a growing 

 feather (Fig. 93). 



The exoskeleton of mammals consists primarily of hair, but scales 

 occur in some; for example, on the tails of rats and of beavers and 

 in the armor of the armadillo. The highest development of mam- 

 malian scales is found in the armadillo armor. It is composed of 

 two layers of scales, the outer scutes of ectodermal origin and the 

 inner bony plates of mesodermal origin. In the human body the 

 only traces of an exoskeleton are the hair, the nails, and the greatly 

 modified scales which form the teeth. The human skin (Fig. 94), 

 like all vertebrate skin, consists of two layers, an outer epidermis 

 of ectodermal origin and an inner dermis, or corium, of meso- 

 dermal origin. The epidermis is made up of a considerable number 

 of cell layers, the innermost of which consists of cells that are 

 actively dividing throughout life. This active layer, termed the 

 STRATUM germixativum, is the source of the hair, of the sebaceous or 

 OIL glands, and of the sudiferous or sweat glands. The dermis, or 

 corium, is composed ot loose connective tissue and contains minute 

 blood vessels, nerve endings, and, in some regions, muscles. The 



