ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 37 



2. The tissues of the connective substance. Under this term 

 there are included a great number of different tissues which morpho- 

 logically resemble each other in the presence of a greater or less 

 amount of intercellular substance, intercalated between the cells (con- 

 nective tissue corpuscles). They connect and surround other tissues, 

 and serve as supporting and skeletal structures. The intercellular 

 substance arises from the cells as a differentiation of the peripheral 

 part of their protoplasm ; it cannot accordingly be genetically clearly 

 distinguished from the cell membrane and its differentiations, which 

 we have considered in connection with epithelial tissue. The cell 

 walls already produced by the protoplasm may also become fused 

 with the intercellular substance, and so contribute to its increase. 

 The intercellular substance is usually secreted by the whole periphery 

 of the cell, and presents great variations both in its morphological 

 and chemical characters. 



When the amount of intercellular substance is small, the tissue is 

 called cellular or vesicular connective tissue. This form is found 

 especially in medusae, molluscs, and 

 worms, and to a less extent in verte- 

 brates (notochord, fig. 25), and is not 

 sharply marked off from cartilaginous 

 tissue. Embryonic connective tissue, 

 which consists of closely aggregated 

 embryonic cells, evidently closely re- 

 sembles it. 



Mucous or gelatinous connective tissue 

 is characterised by possessing a watery 

 hyaline and gelatinous matrix. The 

 condition of the cells in each case is 

 different. Frequently they send out 

 delicate, often branched processes 

 which anastomose with one another 

 and form a network. In addition, 

 however, parts of the intercellular 

 substance may be differentiated into bundles of fibres (Wharton's 

 gelatine in the umbilical cord). Such forms of tissue are found 

 amongst the Invertebrata, e.g., in Heteropods and Medusae, whose 

 gelatinous disc, in consequence of the reduction or complete absence 

 of cells, is reduced to a layer of soft or hardened connective tissue 

 but little different in its origin, as a unilateral cell excretion, from 

 cuticular structures (Hydroid Medusae, swimming bells of Siphono- 



FIG. 25. Vertebra of larva of a toad 

 (after Gotte) . C%, notochord cells ; 

 ChS, notochord sheath ; Sk, skele- 

 togenous tissue ; N, spinal cord. 



