STRUCTURE OF THE VERTEBRATES 



111 



There is no evidence that the body plates were carried over 

 to the mammals, although the bones of the top of the head and 

 face are homologous structures. In certain small whales there 

 are calcified plates on the dorsal surface; and the armadillos 

 (page 87) are well armored with bony shields. Most anatomists, 

 however, consider these cases as secondary developments, not as 

 homologous structures. The evidence is too complicated for a 

 discussion here. 



B. Ectodermal Derivatives of the Skin 



With the exception of the dermal denticles of the elasmo- 

 branchs, there is a clean-cut distinction between dermal and 

 epidermal derivatives of the skin. The difference is based upon 



.Single secretory cells 



land duct 



M^^ OjiMM ^ 



Lamprey Skin 

 (Long. Section) 



Secretory 

 epithelium 



Frog Skin (Section) 

 Simple aciniform gland 



Branching 

 Aciniform 



Simple tubular 



Fig. 48. Gland Types. The skin drawings (top) show the relationships 



with the dermis and muscles. The diagrams (bottom) show the branching 



of simple glands to form more complicated structures. 



