I02 



CHORDATE ANATOMY 



together, the cytoplasm and cell-membrane of each cell being the wall of a 

 bubble. 



The vacuolated notochord tissue is enclosed by sheaths which differ 

 in number and nature in various animals. There is commonly an inner 

 elastic sheath (Fig. 98, ei) composed of material secreted by an outer 

 epithelioid layer of the notochord tissue, and a thick outer sheath of dense 

 fibrous connective tissue. Mechanically, the notochord resembles a 

 length of rubber tubing, closed at the ends, and filled with liquid under 

 pressure. 



Fig. 98. — 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; ci, epithelioid internal elastic sheath of notochord; /, 

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

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

 Kingsley.) 



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 sepa- 

 rated from one another (Figs. 68, 99). The intercellular 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 exceed- 

 ingly fine canals penetrating the matrix and putting any one lacuna into 

 communication with neighboring lacunae. 



