42 



COMPARATIVE ANATOMY 



CHAP. 



outer which covers the surface of the body a homogeneous jelly, con- 

 taining a large proportion of water, is secreted by the epithelium. 

 Cells leave the epithelium and enter the jelly, where they assume a 

 different form. Sometimes they become spindle shaped, sometimes 

 much branched cells connected together by their branches, sometimes 

 elastic fibres. Occasionally such cells show amoeboid movement. 

 They can even become contractile muscle-cells. 



II. The Conneetive tissue propep does not take its rise direct 

 from the epithelium. In early stages of development of the animal 

 variously sized groups of cells sink out of the epithelium below the 

 surface, multiply by fission, and so produce the actual formative cells 

 of the connective tissue. This fills the interstices between the organs 

 and other tissues, or forms pillars, strands, plates, and variously 

 shaped supporting masses. It often forms membranes round other 

 organs and tissues, or lines cavities. Such a superficial extension may 

 even assume the character of an epithelium. 



Two chief types of connective tissue proper are to be distinguished. 



A. The cells of the connective tissue lie close together and form 

 no intercellular or connecting substance. 



Vesicular conneetive tissue (Fig. 37). Vacuoles filled with fluid 

 occur in the cells, which, growing in size, cause vesicular swelling. 







R V 



FiG. ST.— A, Younger, B, older vesicular connective tissue of a Platoae. r. Vacuoles. 



The protoplasm is then often limited to a thin layer surrounding the 

 vacuole, and this can mingle with the neighbouring cells. Small 

 aggregations of protoplasm may still be found massed round the nuclei. 



Vesicular connective tissue passes into reticular connective 

 tissue, when the fluid- filled spaces of neighbouring cells unite ivith 

 one another. The connective tissue then takes the character of a 

 spongy network containing imbedded nuclei, and the intracellular fluid 

 which has flowed together becomes in a certain sense intercellular 

 (reticular connective tissue of many Platodes). In fatty tissue (Fio-. 

 38) smaller or larger fat drops appear in the protoplasm of the cells. 

 In pigment tissue colouring matter is deposited. 



B. The formative cells of conneetive tissue form externally a 

 substance, the intercellular substance, in which they come to lie em- 



