CYTOLOGY OF REPliODLCTlON IN ANIMALS 135 



become macronuclei and four micronuclei. By two succeeding fissions the four 

 macronuclei are then distributed, one to each of the four resulting iiKhvichmls. 

 In some other species the micronuclei are equallj^ distributed in like manner, 

 but in P. caudatum the process is moye complicated, since three of them degener- 

 ate, and the fourth divides twice to produce four new micronuclei. In either 

 case at the close of the process each of the conjugating individuals has given rise 

 to four descendants, each containing a macronucleus and a micronucleus derived 

 from the cleavage-nucleus. From this time forward fission follows fission in the 

 usual manner, both nuclei dividing at each fission, until, after many generations, 

 conjugation recurs. (Wilson.) 



In these Infusoria there is also another process, endomixis, in which 

 the nuclear reorganization closely parallels that in conjugation: it resem- 

 bles autogamj^ in occurring entirely within one individual, but it differs 

 from both autogamj^ and conjugation in involving no nuclear fusion. If 

 the first two divisions of the original micronucleus here accomplish a 

 reduction in chromosome number, as they evidently do in conjugation, 

 it would seem that the diploid number should be restored sooner or later. 

 This point has not been cleared up by cytologists. Both conjugation 

 and endomixis (in species having it) have beneficial effects upon the 

 vigor of the race. In cultures the organisms continue to multiply by 

 fission as long as either process occurs at intervals, but when conditions 

 inducing senile change arise, their vitality decreases and the cultures 

 die out if neither process takes place. Finally, it is of interest to know 

 that researches on Paramecium and other genera indicate the presence 

 of a genie basis of inheritance among Infusoria paralleling in some degree 

 that found in Metazoa. The problem of analyzing the experimental 

 data is, however, greatly complicated by the occurrence of several types 

 of nuclear reorganization, the presence of macronuclei, and other factors. 

 Intensive studies now in progress promise to show to what extent genet- 

 ical principles founded on data derived from higher animals and plants 

 are applicable to Protozoa. 



With the above points in mind it becomes unsatisfactory to regard 

 Protozoa merely as simple or primitive organisms. They are small, but 

 not therefore simple. Their organization may not be so complex as that 

 of frogs, but for animals of their size it is probably fully as effective. 

 Which of their peculiar characters are actually primitive can scarcely 

 be stated with confidence at present. Perhaps it is best to think of 

 them as did their discoverer, Anton van Leeuwenhoek, simply as "little 

 animals." 



