PARAMECIUM 45 



although there is plenty of food, the animals are not able to repro- 

 duce as rapidly as usual, due, supposedly, to some internal fac- 

 tor or factors. A depression of this type is the external evidence of 

 the beginning of a periodic process of intracellular reorganization 

 and adjustment between the cytoplasm and nuclear material in 

 each animal, known as endomixis. 



The details of the process of endomixis in the species known as 

 Paramecium aurelia, which normally has two micronuclei, may be 

 outlined as follows : 



(1) Each of the micronuclei divides twice to form a total of 

 eight micronuclei. (W. f. 171, A, B, C.) 



(2) The macronucleus fragments and finally is completely 

 broken down and dissolved in the cytoplasm. (W. f. 171, B.) 



(3) Six out of eight of the newly formed micronuclei degenerate. 

 (W. f. 171, D.) 



(4) The cell divides, and one micronucleus goes to each daughter 

 cell. (W. f. 171, D, E.) 



(5) The micronucleus in each cell divides twice to form four 

 nuclei, two of which become macronuclei and two become micronu- 

 clei. (W. f. 171, F, G.) 



(6) Each of the two cells now divides, thus giving four normal 

 cells. (W. f. 171, /, J.) 



From the above description, it can be seen that the process of 

 endomixis brings about a complete, periodic reorganization of the 

 entire nuclear apparatus of an animal and a new adjustment 

 between cytoplasm and nucleus. When the process has been 

 completed, the normal rate of division, which has been temporarily 

 depressed, is again established, and the animal — if the environ- 

 mental conditions are suitable — can grow and reproduce by 

 binary fission with the normal rapidity for some weeks until the 

 onset of the next endomictic period. 



Conjugation. Considering next the process of conjugation, 

 which is of very general occurrence among the Protozoa, it should 

 be emphasized that, while it is a process of nuclear and cytoplasmic 

 reorganization, it also involves, first, an interchange of nuclear 

 material between two animals and, second, the permanent fusion 

 (fertilization) in each animal of the nuclear material received 

 with a portion of the nuclear material already there to form a 

 composite nucleus, or synkaryon. 



The first step in the process of conjugation in Paramecium is 



