UNICELLULAR ORGANISMS 67 



dividing with each subsequent cell division. Apparently the 

 purpose of this conjugation is an interchange of the material 

 present in the micronucleus; for it will be seen that after con- 

 jugation each of the resulting animals contains nuclear material 

 derived from the micronucleus of the other individual as well 

 as from its own 



The Life Cycle of Paramecium. We usually think of the life 

 history of higher animals as marked off in definite life cycles. 

 For example, from the egg of the hen develops the chick, which 

 grows into an adult hen and produces another egg and thus 

 starts the process over again. Such a life cycle we speak of as 

 comprising a single generation, and by the term individual we 

 refer to all the stages of the life of the organism between one 

 point in the cycle and the next similar point. When we attempt 

 to think of the Paramecium in a similar way, we find the case 

 so modified that the terms are somewhat difficult to apply. 

 But still in the Paramecium we can recognize a life cycle some- 

 what similar to that of other organisms. We shall learn in a 

 later chapter that the life of an animal like a hen begins with 

 a single cell, which, dividing by a process similar to that we have 

 just studied in the Paramecium, gives rise to a large number of 

 cells; see Fig. 15. These, however, remain attached to form 

 the individual which we speak of as the chick, which grows into 

 the hen, and which is thus composed of large numbers of cells. 

 This individual continues a separate existence and eventually a 

 single cell is separated from it to form another egg and to start 

 the process over again, in a new individual. 



Now if we compare these facts with those just seen in the 

 Paramecium, we shall find that the life cycle of the Paramecium 

 is as follows: Starting in the cycle at the point where two ani- 

 mals separate after conjugation, there begins a series of cell 

 divisions which rapidly increases the number of cells. The cells 

 at once separate from each other, become perfectly independent, 

 swimming apart as quite isolated animals, In this respect the 

 development of the Paramecium differs very markedly from 



