CELL DIFFERENTIATION 15 



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. 



It is obvious that the tissues and organs so derived will exhibit in a varying 

 degree the primary properties of protoplasm. The muscles, for example, 

 derived chiefly from certain cells of the mesoblast are particularly contractile 

 and respond to stimuli readily, while the cells of the liver, although possibly 

 contractile to a certain extent, have to do chiefly with the processes of 

 nutrition. 



As the cells of the embryo increase in number in development there is a 

 corresponding differentiation of function among the groups of cells. The 

 various functions which the original cell may be supposed to discharge, and 

 the various properties it may be supposed to possess, become divided among 

 groups of cells in which the work of each group is specialized. As a result 



FIG. 13. 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.) 



of this division of labor the functions and properties are developed and made 

 more perfect, with a view to the more economic and effective accomplishment 

 of the activities of the body as a whole. 



In studying the functions of the human body it is necessary first of all to 

 know of what it is composed, of what tissues and organs it is made up; this 

 can of course be ascertained only by the dissection of the dead body, and thus 

 it comes that Anatomy, the science which treats of the structure of organized 

 bodies, is closely associated with physiology, which treats of the function of 

 the same structures. So close, indeed, is the association that Histology, 

 which is especially concerned with the minute or microscopic structure of the 

 tissues and organs of the body and which is, strictly speaking, a department 

 of anatomy, is often included in works on physiology. There is much to be 

 said in favor of such an arrangement, since it is impossible to consider the 

 changes which take place in any tissue during life, apart from the knowledge 

 of the structure of the tissues themselves. There is indeed an almost insep- 

 arable relation between the structure and the function of the differentiated 



