k H 4 ELEMENTARY ZOOLOGY. 



the internal skeleton of cartilage or bone, the connective or binding 

 tissues, the blood and blood vessels, and the internal reproductive 

 organs. In producing these organs two or more layers may be 

 associated, as the ectoderm and mesoderm in forming the skin, or 

 the entoderm and mesoderm in the digestive tract. In the latter 

 the entoderm furnishes the lining which is digestive and absorptive ; 

 the mesoderm forms the connective tissue and the muscle fibres 

 that cause contractions in it. 



FIG. 21. A diagram illustrating one of the modes of forming mesoderm from the 

 entoderm of the gastrula. A, early stage; B, later stage, a, archenteron; bp, blasto- 

 pore; cos, body cavity, formed from pockets of the archenteron; ec, ectoderm; en, ento- 

 derm; m, mesoderm; m.so., mesoderm of the body wall; m.sp., mesoderm of the viscera; 

 s.c., segmeutation cavity. 



Questions on the Figure. What layer is most active in forming 

 the mesoderm here? From what cavity is the coelom produced? 

 In what cavity does the mesoderm form? What finally becomes of 

 the segmentation cavity? Compare figures in other texts and find in 

 what other ways mesoderm is formed. Draw a diagram, by reference 

 to other books, of a stage later than B. 



124. Tissues. In the differentiation referred to above the cells 

 in the various parts of the body are becoming quite different from 

 each other. In each of the complex organs there may be several 

 kinds of cells that differ considerably in shape, size, and other 

 respects. Groups of similar cells, differentiated from the other 

 cells about them, are called tissues. It is well known that these 

 groups of cells become especially fitted to do certain kinds of work : 

 some are specially sensitive; others are contractile; some manu- 

 facture and pour out secretions that influence the general condition 

 of the body; and some produce hard and durable substances which 

 furnish protection and support. 



In this division of labor lies the advantage which the complex 



