GENERAL ANATOMY. 85 



protozoa which are rich in chromatin. Accordingly the 

 chromatin contents and the degree of organization of the 

 cell must condition the different character of the processes 

 of nuclear division. As mentioned above, there is reason 

 to regard chromatin as the bearer of heredity. Now the 

 more highly an animal is organized, so much the more are 

 peculiarities transferred from cell to cell, so much the more 

 complicated must be the structure of the hereditary sub- 

 stance, the chromatin, so much the more delicately elabo- 

 rated, therefore, must be those processes which condition 

 the definite distribution of the hereditary substance to the 

 daughter-cells. The significance of indirect division can 



O O 



only be sought in this, that the hereditary substance present 

 in small amount must be handed over to the daughter-cells 

 in equal quantities and of the same constitution. This 

 object is simplified in the case of the lower animals, in 

 which much chromatin is present, and this, too, of a simple 

 composition. 



II. THE TISSUES OF THE ANIMAL BODY. 



Definition of Tissue. In the formation of tissues two 

 processes are operative : (i) the multiplication of cells by 

 means of division into cell-complexes, and (2) the histo- 

 logical differentiation of cells. A tissue, therefore, can be 

 defined as a complex of differentiated cells Jiistologically 

 similar. 



Nature of Histological Differentiation. The histo- 

 logical differentiation is chiefly expressed in this, that the 

 cells have definite form and definite position relative to the 

 neighboring cells; in addition, there almost always comes 

 in, as a second and more important feature, the histological 

 alteration of the cell. The fact has already been men 

 tioned that the cell uses its food-material, not only for its 

 own growth, for increase of its protoplasm, but also, in 

 another manner, for forming substances, protoplasmic pro- 

 ducts, either in its interior (internal plasmic products), or 

 more often on its surface (external plasmic products). Now 



