44 MANUAL OF ANIMAL BIOLOGY 



surrounding endoplasm. The two vacuoles, as a rule, contract 

 alternately. 



3. Reproduction 



In Paramecium, just as in the other unicellular forms previously 

 noted, reproduction occurs by binary fission, that is, by the mitotic 

 division of the cell into two daughter cells. Paramecium and most 

 other ciliated forms divide transversely, whereas in Euglena and 

 other Flagellates it will be remembered that the division is longi- 

 tudinal. (W. f. 28.) 



During the process of cell division, which has been very com- 

 pletely studied in Paramecium, there is, first, a division of the 

 micronucleus into two equal parts. Then the macronucleus, hav- 

 ing elongated somewhat, also divides into halves. Following this, 

 the cytoplasm in the center of the animal begins to constrict trans- 

 versely. The constriction continues to deepen, and in the course 

 of an hour or so, the animal is completely divided into two daughter 

 cells, each of which, although at first only one-half the normal size, 

 nevertheless contains the normal nuclear structure and most of the 

 other characteristic features. Structural modifications are occur- 

 ring synchronously with the division of the nuclei and the cell 

 body, so that when the two halves separate, the animal formed 

 from the anterior half has a newly formed posterior end, and the 

 animal resulting from the posterior half has a newly formed 

 anterior end with the various specialized structures, such as the 

 peristome and contractile vacuole. This process of cell division 

 normally may take place two or three times within twenty-four 

 hours. 



The life history of Paramecium, however, is not so simple as 

 might be thought from the above description of reproduction by 

 binary fission, for it has been shown that after reproduction has 

 gone on for a considerable number of generations in this man- 

 ner, a reorganization of the cytoplasmic and nuclear material 

 typically occurs. It is probable that this protoplasmic reorgani- 

 zation can take place either by endomixis or by conjugation. 



Endomixis. If Paramecia are kept under certain culture condi- 

 tions in which they can be observed day by day, it will be found 

 that a lowering of the division, or reproduction, rate occurs at 

 quite regular intervals. This reduction is apparently independent 

 of the food or of other environmental conditions. In other words, 



