154 



COMPARATIVE ANATOMY 



collectively forming an alveolar framework for the inner tissue. It must, 

 however, be due largely to the enclosing sheaths which, taken all together, 



Fig. ii6. — Developing vertebrae of the amphibian, Ambystoma; I, earlier; II, later. 

 Longitudinal sections. Cartilage and bone are forming around the notochord. cc. 

 cartilage in center of vertebra; ei, epithelioid internal elastic sheath of notochord; i, 

 incisure cutting through ic, intercentral (intervertebral) cartilage; w, notochord; ns, outer 

 notochordal sheath; v, developing bone (black) of centrum of a vertebra. (From 

 Kingsley.) 



are tough and not highly elastic. Mechanically, the notochord resembles a 



length of rubber tubing, closed at the 

 ends, and filled with liquid under 

 some degree of pressure. 



Cartilage. In development of 



cartilage, mesenchyme cells become 



densely massed and then produce 



an abundant intercellular substance 



whose accumulation causes the cells 



to become more or less widely 



separated from one another (Figs. 



76, 117, 118). The intercellular 



substance, the matrix, becomes 



solid and acquires a firm or even 



hard consistency. Chemically it is a 



complex of collagenous, albuminoid and other protein substances. The 



cartilage cells remain imbedded in the matrix, each occupying a 



close-fitting space, a lacuna. In some cartilages have been described 



Fig. 117. — Hyaline cartilage with 

 cartilage cells lying in lacunae enclosed 

 by the matrix. Thin section highly 

 magnified. (From Kingsley.) 



