232 



PRACTICAL ZOOLOGY 



of protoplasm. Volvox is particularly interesting because it 

 illustrates very clearly the division of labor between cells that 

 are set aside for reproduction, the germ cells, and those that 



carry on the rest of the activities 

 of the animal, the body cells or 

 somatic cells. Certain repro- 

 ductive cells of the Volvox 

 colony grow very large, divide 

 into a number of cells, and thus 

 form new colonies. Other re- 

 productive cells increase in size, 

 some of them becoming eggs, and 

 others dividing to form many 

 spermatozoa 

 (Fig. 137, B, $ 

 and 9). The 

 eggs are fer- 

 tilized by the 

 spermatozoa, 

 and after a 

 period of rest 

 develop into 

 new colonies. 

 The new col- 

 onies thus 

 formed escape 

 from the par- 

 ent colony and 

 continue the 

 race, whereas 

 the old col- 

 onies die a nat- 



FIG. 137. Colonial protozoa. 

 A, uroglena ; B, volvox. (After Kolliker.) 



ural death. It is evident that unless germ cells were formed 

 the Volvox race would soon disappear, and it is also clear that 

 there is here a continuity of germ cells from one generation to 



