TISSUES. 363 



Connective tissue (connectivum) must be regarded as 

 forming, in order of phylogenetic age, the second main 

 group of tissues. This is morphologically characterized by 

 the intercellular substance, which develops between the 



PIG. 291. Jelly-like tissue from the vitreous body of an embryo of four 

 months (round cells as jelly-like intercellular substance). 



FIG. 292. Cartilage-tissue of the fibrous or netted cartilage of the ear- 

 shell : a, cells ; b, intercellular mass ; c, fibres in the latter. (After Frey.) 



cells, physiologically, by the double part which it plays, 

 as connecting substance and as complementary substance 

 between the other tissues, as an inner supporting substance 

 and as a protective covering for the inner organs. Of the 

 numerous forms and varieties of connective tissue, we regard 

 the jelly-like tissue (Fig. 291 ; Fig. 6, voL i. p. 126), the fatty 

 tissue, and the chorda tissue as the earlier; the fibrous, 

 cartilaginous (Fig. 292), and bone-tissue (Fig. 5, voL i. p. 126) 

 as the more recent formations. All these various forms of 

 connective tissue are products of the middle germ-layer, 

 or mesoderm ; or, more accurately, of the two fibrous layers, 

 of which the skin-fibrous layer is originally derived from 

 the exoderm, the intestinal-fibrous layer from the entoderm. 

 The nerve-muscular tissue (neuro-musculum) is of much 

 more recent origin than the connective tissue. If epithelial 

 tissue represents a primary period in tribal history, and 



