76 



PROTOZOA 



to fertilization in higher animals which possess sex, the smaller mieronu- 

 cleus being viewed as male and the larger one as female. However, it 

 cannot, properly speaking, be called fertilization, since no gametes are 

 involved. The macronucleus begins to degenerate soon after the micro- 

 nucleus leaves it, breaking up into fragments. 



The two animals now separate and the fragments of the macronuclei 

 slowly disappear, their substance being dissolved in the endoplasm. 

 The fusion micronucleus in each of the exconjugants divides by mitosis 

 into two, these into four, and these into eight, all equal in size. Four of 



1. Animal before endomixis begins 



Micronuclei divide. Macronucleus disintegrates 



3. Micronuclei again divide 



Six out of eight micronuclei disappear. Animal divides 



Each animal with one micronucleus 



Micronucleus of each divides 



7. Micronuclei divide again. 



S. Two micronuclei in each animal become 

 macronuclei 



Micronuclei divide and animals divide 



Four ordinary individuals 



Fig. 28. — Diagram illustrating endomixis in Paramecium aurelia Miiller. (From Wood- 

 ruff, "Animal Biology," by the courtesy of The Macmillan Company.) Large black bodies 

 are macronuclei, small ones micronuclei, and small circles micronuclei which disappear. 



these then become larger and develop into macronuclei. Of the four 

 remaining, three disappear. The fourth divides into two, and the animal 

 divides, each of the two individuals formed having one micronucleus and 

 two macronuclei. In each individual the micronucleus again divides and 

 this is followed by fission, producing four animals each with one micro- 

 nucleus and one macronucleus. This description applies to Paramecium 

 caudatum; the steps in the process are variously modified in other species. 

 The significance of conjugation is uncertain. Some investigators 

 believe that after a long series of fissions the animals become senescent 

 and conjugation serves as a process of rejuvenation which restores their 

 vitahty. Woodruff, however, has succeeded in maintaining a culture 

 since May 1, 1907, without conjugation. In the thirty-second year, 



