24 INTRODUCTION. 



their potential qualities are quite diverse. As growth proceeds, 

 they begin to vary in size, shape, and internal structure in the dif- 

 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. In this elaboration of the 

 bodily organs, cells from the different embryonic layers enter into 

 very intimate cooperation, nearly all the organs of the body con- 

 taining structures derived from all three layers; thus, the lungs, 

 for example, contain epithelium which is descended from the ento- 

 derm ; fibrous tissue, cartilage, and bloodvessels from the mesoderm, 

 and nerves and ganglia arising from the ectoderm. 



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 



tiSBUeS. 



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 thai the cells exercise their most marked formative 

 power- during the development of the body, causing the deposition 

 of intercellular substances which possess the requisite physical char- 

 acter — 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. 



