THE VERTEBRATE BODY 



135 



two chief layers: an outer epithelial tissue, the epidermis, derived 

 from the ectoderm, and an inner dermis from the mesoderm of the 

 embryo. 



The epidermis itself always consists of several layers of cells : the 

 lower comprising actively dividing cells whose products gradually 

 are moved up to form the superficial layers of flattened, horny cells. 



Enamel 



Dentine 



Crown < 



— Gum 



Neck 



Root < 



Periosteum 

 Cement 



Wj 



$tl! — Root canal 



Nerve 



Fig. 97. 



Human tooth, longitudinal section. 



Thus this layer is not directly converted into the cuticle, as is the 

 case, for example, in the Arthropods. The dermis is a connective 

 tissue layer, with glands, blood vessels, etc., between the epidermis 

 and the muscular layer of the body wall. (Figs. 96, 97.) 



C. Muscles 



The body wall proper is chiefly composed of muscular tissue, 

 commonly spoken of as flesh, which varies in thickness in dif- 

 ferent parts of the body. In the mid-dorsal region it surrounds the 

 brain and spinal cord (central nervous system) and the axial 

 supporting structure (notochord), while ventrally it forms the 

 wall of the coelom. In the lower Vertebrates and the embryonic 



