DIFFERENCES BETWEEN ANIMALS AND PLANTS 



II 



it is found that from each of these layers is produced a very definite part 'of 

 the completed body. For example, from the cells of the epiblast are derived, 

 among other structures, the skin and the central nervous system; from the 

 mesoblast the muscles and connective tissue of the body, and from the 

 hypoblast the epithelium of the alimentary canal, some of the chief glands, 

 and so on. 



The result of this developmental process therefore is the formation of the 

 adult tissues highly differentiated and specialized in form. 



From the physiologist's point of view this anatomical differentiation 

 accomplishes a highly specialized structure which, machine-like, is capable of 

 doing some part of the total work of the body in an especially effective manner. 

 In a word, the differentiated tissues do not altogether lose the general proper- 

 ties which characterize protoplasm, but each tissue develops a structure capa- 

 ble of doing some special part of this activity better than the undifferentiated 

 protoplasm can do it. As an illustration, the muscles, derived chiefly from 



FIG. 10. Transverse Section through Embryo Chick (26 hours), a, Epiblast; b, 

 mesoblast; c, hypoblast; d, central portion of mesoblast, which is here fused with epiblast; 

 e, primitive groove; /, dorsal ridge. (Klein.) 



either epithelial cells or mesoblast, are highly contractile, and especially 

 responsive to stimuli. They have not developed in the same degree the power 

 to produce chemical substances which characterizes the salivary glands. The 

 cells of the liver, on the other hand, in the adult stage have practically lost the 

 property of contractility, but have developed in a high degree the functional 

 properties of nutrition and secretion. 



Hand in hand with the anatomical differentiation has gone physiological 

 division of labor. In the adult animal body each type of activity is no longer 

 accomplished by the whole organism as in the case of the ameba, but now 

 some specializing part of the body assumes each chief activity. In other 

 words the muscles contract, the nervous tissue conducts changes from one part 

 of the body to the other, glands secrete, special sense organs respond to the 

 stimuli of the environment, and the reproductive gonads have assumed the 

 chief responsibility in the reproductive process. It is through the high 

 degree of physiological division of labor that the great versatility of the body 

 activity is accomplished by the human organism. It is a matter of great 

 economy and effectiveness in this biological machine, the body. 



