148 GENERAL ZOOLOGY 



differentiation into soma and germ had, and still has, one 

 disadvantage, however all the somatic cells must die when 

 the body is worn out. 



After germ and somatic cells were established another 

 improvement came in, by dividing the labor between the 

 sexual cells. As stated in the last chapter a young indi- 

 vidual is produced by the fusion of two sexual cells to form a 

 single cell. It is desirable, if two sex cells are going to fuse, 

 to have them able to move so that they can more readily 

 get together. It is also desirable that they contain reserve 

 energy, so that development may go on after fusion has 

 taken place, during the time when the usual activities are 

 more or less disturbed. Locomotion and storage of nour- 

 ishment are therefore two essential qualities in germ cells. 

 It is not very feasible, however, to have a cell store large 

 quantities of food and at the same time be able to move 

 about readily. This difficulty has been met by the develop- 

 ment of two kinds of germ cells: (1) ova, which are large, 

 immobile, and contain much nourishment ; and (2) sperma- 

 tozoa, which are minute and very active. 



In a number of groups of protozoans colonies of cells 

 have become differentiated to the extent described up to 

 this point, but only true metazoans have differentiated 

 the somatic cells so that tissues are formed. Tissues are, 

 therefore, the only criterion which will separate one group 

 from the other. If, in the future, some colony of protozoans 

 develops a division of labor in its somatic cells, so that one 

 group attends to one function and another group to some 

 other activity, we will then be obliged to classify it as a 

 many-celled animal. 



TISSUES AND ORGANS 



A tissue is a group of similar cells w T hich carries on some 

 particular function in a metazoan's body. Tissues have 

 developed chiefly along four lines, and each of the chief 

 types has arisen by emphasizing some quality present in all 

 protoplasm. The groups are as follows: 



