INTRODUCTION 



Zoological research of the last decade has led to a sharp 

 separation of two chief divisions of the animal kingdom: 

 the Protozoa and the Metazoa. In the group Protozoa the 

 individual can, from its structure, be referred to the funda- 

 mental type of a cell. These unicellular individuals exist 

 either separately or united in great numbers to form colonies 

 or corms. In the latter case, however, the different indi- 

 viduals remain equivalent to one another in structure and 

 function. In the group Metazoa., or Germ-layer animals., 

 on the contrary, there always results a multicellular or- 

 ganism (cell-community or cell-corm), in which the single 

 cells give up their independence for the good of the com- 

 munity, and accommodate themselves to a division of labour, 

 in consequence of which there is brought about a diversity 

 in the structure and function of the cells of the Metazoan 

 organism. While the development and differentiation of 

 distinct tissues with specific functions result from this poly- 

 morphism of the cells, the entire colony gains a higher 

 functional capacity and a more complete unity. In this 

 way there arises an individual of higher rank or second 

 degree, which we designate as person. These Metazoan in- 

 dividuals also may, through incomplete separation after 

 budding, remain united in colonies, and then there results 

 an individual of the third degree, the stock or corm. By 

 adaptation of the stock-forming persons to various functions, 

 accompanied with their polymorphous development, a higher 

 functional unity may be reached in this case also. 



As a result of the division of labour which is effected 

 among the cells of the Metazoan organism, it comes about 



K. H. E. ^ ß 



