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tissue which, judging from its origin and chemical properties, 

 is related to epithelium. On the other hand, the fact that it 

 may be transformed into cartilage would seem to bring it nearer 

 to the connective tissues. 



II. SUPPORTING, CONNECTING, AND INTERSTITIAL TISSUE. 



This group is made up of tissues whose function it is to form 

 the supporting framework for the organs and for the body ; to 

 join together the units which make up the organs ; and to fill 

 up the spaces between such units and organs. 



A general characteristic of these tissues is the presence of a 

 large quantity of intercellular or ground substance, so that the 

 cellular elements are often inconspicuous. The connecting sub- 

 stances are distributed throughout the whole body, and are 

 classified mainly with regard to physical and chemical differ- 

 ences in the intercellular substances. We distinguish : 1, con- 

 nective tissue ; 2, cartilage ; and 3, bone. 



Usually these tissues can plainly be distinguished from one 

 another. They are grouped together : because they are closely 

 related both ontogenetically and phylogenetically ; because when 

 they are near one another there is often no sharp line to be 

 drawn between them ; and because they are capable of replacing 

 one another. So we see, for example, that the skeleton in the 

 different classes of animals may consist of soft connective tissue, 

 of cartilage, or of bone. Similarly the sclera in higher animals 

 is a connective-tissue structure, while in some fishes it is bony 

 or cartilaginous. Also it is well known that bone may develop 

 from cartilage, and that cartilage may develop connective-tissue 

 fibres in its substance. All of these tissues are of raesodermal 

 origin i. e., they arise from the middle germinal layer (meso- 

 derm). 



These tissues begin to develop from the so-called embryonic 

 cellular tissue. This consists of round or polygonal cells with, 

 in the beginning, no ground substance. Later the cells change 

 their form and become spindle-shaped, or, by the formation of 

 anastomosing processes, stellate. At this time the cells lie in a 

 semifluid intercellular substance, which is certainly a product 



