24 INTR OD UCTION, 



ferent parts of the foetus. Their relative positions become modi- 

 fied. The primitive organs are defined and the tissues of which 

 they are composed become elaborated. 



The elaboration of the tissues is wrought by the cells, which dis- 

 play what is called their formative powers in the production of 

 materials of various sorts which lie between them, and are called 

 the intercellular substances. The amount and kind of intercellular 

 substance vary, each form of tissue having its own peculiarities in 

 this respect, dependent upon the role it is to play in the general 

 economy. Some of the tissues perform functions which require the 

 active processes that can be carried on only in cells, and in these 

 the intercellular substances are either small in amount and appar- 

 ently structureless, as in epithelium, or their place is taken by a 

 tissue of separate origin, while the cells, relieved of the necessity 

 for exercising their formative powers in this direction, become 

 highly specialized to meet the functional demands imposed upon 

 them. This development is met with in the muscular and nervous 

 tissues. 



Other tissues of the body are of use mainly because of their 

 physical properties, such as rigidity, elasticity, tensile strength, plia- 

 bility, etc. These tissues, collectively called the connective tissues, 

 are essentially passive. They require little or no cellular activity 

 for the performance of their functions, and it is in the elaboration 

 of these tissues that the cells exercise their most marked formative 

 powers during the development of the body, causing the deposition 

 of intercellular substances which possess the requisite physical char- 

 acters rigidity and elasticity in the case of bone, pliability and ten- 

 sile strength in the case of ligamentous structures, etc. As these 

 substances are perfected, the cells decrease in activity, until they 

 merely preside over the integrity of the intercellular substances they 

 have already produced. 



It may be well to point out here a distinction that divides the 

 tissues of active cellular function into two groups. The first group, 

 including the various modifications of epithelium, displays its ac- 

 tivity in the elaboration of material products, taking the form of 

 either new cells which are continually being produced, or of certain 

 chemical substances which appear as a secretion. The second 

 group, comprising the muscular and nervous tissues, exercises its 

 functional activities in the storage of latent energy in such sub- 

 stances of unstable chemical nature and in such a manner that it 



