MORE COMPLICATED LIFE CYCLES 113 



isolation. In the reorganization of the cell following separation two 

 micronuclei instead of one were left in the cell. This abnormality for 

 the "caudatum" form was the "normal" condition for the "aurelia" 

 form, and was maintained for more than three months, the animals 

 showing every characteristic of form and function that have been 

 ascribed to Paramecium aurelia. The movement was sluggish and 

 the rate of division much lower than in the case of "caudatum" forms 

 which had been isolated at the same time and carried along as a con- 

 trol (see Fig. 40). At the expiration of three months in culture the 

 cells here and there showed the loss of one of the micronuclei, and ulti- 

 mately all of the so-called "aurelia" forms had become "caudatum" 

 forms and with the typical characteristics which mark this species. 

 The rate of division rose to a much higher average than before, 

 and the cells became much more animated and larger in size. The 

 average number of divisions in ten-day periods rose from 11.3 from 

 March 1 to June 10, to 19.3 in the time from June 10 to September 1, 

 that is, during the time when the nuclear relations were normal. It 

 is evident, therefore, that Paramecium caudatum and Paramecium 

 aurelia are not distinct species but merely variants of the same species, 

 and that the abnormal condition of the cell organs resulted in strongly 

 marked physiological derangement. 



B. The Period of Maturity. — There is no definite limit to the 

 period of youth in protozoa, the changes which characterize the period 

 of maturity coming on slowly and imperceptibly as they do in higher 

 forms. The morphological characteristics of this period, when 

 arrived, are clearly marked, however, and unmistakable. Such 

 changes affect both the cell body and the nucleus, and may accom- 

 pany either vegetative or germinal activities, or both. 



1. Protoplasmic Changes at Maturity. — While the most important 

 characteristic of the period of maturity is a general decrease in func- 

 tional activity, with decline in the rate of multiplication, these physio- 

 logical activities are accompanied by well-marked morphological 

 changes which may be of a sexual character. In a single cell or 

 specimen of a protozoan species there may be no clue to its position 

 in the life cycle unless it is in some phase of sexual activity, and where 

 sexual dimorphism does not exist it is quite impossible to tell from 

 morphology alone. Thus, in the mature paramecium the sexual 

 differences are so minute that unless one is following out the life his- 

 tory in culture the period of maturity passes unobserved. Nevertheless, 

 the cells of paramecium do undergo a physical change at this period; 

 the peripheral protoplasm becomes sticky and highly miscible, so that, 

 in some cultures, two organisms upon meeting will adhere at any 

 point, and groups of from six to nine cells may be seen whirling about 

 in aimless movement among the normally conjugating pairs. This 

 miscible state indicates a well-marked difference in the physical 



