XIII 



PHYLUM CHORDATA 



79 



cement. The main bulk of the tooth is made of dentine (Fig. 724, 

 B, ZB), which occurs under three forms. Hard dentine consists of 

 a matrix of animal matter strongly impregnated with lime salts 

 and permeated by delicate, more or less parallel, tubules con- 

 taining organic fibrils. Vaso-dcntine is permeated with blood- 

 vessels, and consequently appears red and moist in the fresh 

 condition. Osteo-dentine approaches in its structure and mode of 



BM 



G 



FIG. 72?. A, semi-diagrammatic transverse section of the intestine of a Craniate ; B, two 

 epithelial cells, highly magnified. B, visceral layer of peritoneum ; D, tubular glands ; 

 E, columnar epithelium (magnified at B, a) ; E 1 , the same with amoeboid processes (magnified 

 at B, b) ; G, G 1 , blood-vessels ; L, lymph-follicles; Li L3, Ly, lymph-cells; Linn, lacteals ; 

 M, longitudinal muscular layer ; M', circular muscular layer ; N, nutritive matters in cavity 

 of intestine being ingested by wandering lymph-cells ; Sa, striated border of epithelial 

 cells; z, sub-mucosa ; Zv villus. (From Wiedersheim's VerttLrata.) 



development to bone: The free surface of the tooth is usually 

 capped by a layer of enamel (ZS), a dense substance, either 

 structureless or presenting a delicate fibrillation, containing not 

 more than 3 to 5 per cent, of animal matter, and being therefore 

 the hardest tissue in the body. The cement (ZC) coats that 

 portion of the tooth which is embedded in the tissues of the jaw, 

 and sometimes forms a thin layer over the enamel ; it has prac- 

 tically the structure of bone. At the inner end of the tooth there 



