188 REPRESENTATIVE SINGLE-CELLED ANIMALS 



and the stationary nucleus with the nucleus of an egg. In this 

 way conjugation is compared with reciprocal fertilization by two 

 hermaphroditic animals. This is a most confusing comparison 

 unless one has an extensive knowledge of the process of syngamy 

 in both protozoa and metazoa. 



In addition to conjugation, there has been discovered, first in 

 Paramoecium aurelia and later in P. caudatum, the process of 

 endomixis, which involves nuclear reorganization within the limits 

 of a single cell, whereas conjugation involves two cells (Fig. 100). 

 As with conjugation, however, there occurs during endomixis a 

 disappearance of t he macronucleus and of a considerable portion of 

 micronuclear material. From the single micronucleus that 

 remains in each cell, a new macronucleus and the two micronuclei 

 characteristic of P. aurelia are then formed. Endomixis has, 

 therefore, some resemblance to conjugation, although only one 

 cell is concerned. Its significance in the life. cycle is probably 

 somewhat the same as that of conjugation. 



There has been much discussion among investigators regarding 

 the significance of conjugation and endomixis. It was originally 

 supposed (Maupas) that conjugation must occur periodically 

 among the Infusoria and that it exercised a " rejuvenating " effect 

 upon paramoecium, since the rate of cell division was described as 

 more rapid just after conjugation and gradually declining until 

 the animal was rejuvenated by another conjugation. Later work 

 has shown that conjugation is not necessary in the life cycle 

 of paramoecium, since P. aurelia has been carried for many 

 thousands of generations without such union of the cells (Wood- 

 ruff). Endomixis has also been shown to be unnecessary, since 

 P. calkinsi has been carried for four years (Spencer) during which 

 time neither endomixis nor conjugation has occurred. 



What, then, is the significance of conjugation in these ciliated 

 protozoa? The present state of our knowledge seems to justify 

 the following general answer: The ciliate L'roleptus, as studied by 

 Calkins, does seem to be rejuvenated, in the sense that a declining 

 rate of cell division is restored to normal by conjugation. Endo- 

 mixis seems also to have this effect in some cases. But varied 

 enviromnental conditions, such as changes in food supply, may also 

 maintain a species at a proper level of cell division and hence 

 normal metabolism. Moreover, there is the case of P. calkinsi, 

 just cited, where neither conjugation nor endomixis seems neees- 



