194 



THE PROTOZOA 



The two conjugants fuse either temporarily or permanently, and 

 the external structures, such as the membranelles, are absorbed. If 

 the fusion is temporary, as in the majority of forms, the two ecto- 

 plasms fuse at or near the mouth parts, and a protoplasmic bridge is 

 formed between the two organisms. The two organisms then become 

 sluggish, and rest for a considerable time upon the bottom without 

 movement of any kind. They ultimately separate and begin to 

 divide. 



The micronuclei play the most important part in conjugation. 

 Each divides two or more times to form four or more daughter-nuclei, 



some of which de- 

 generate, while one 

 divides again, one 

 half to fuse with a 

 similarly derived nu- 

 cleus of the other 

 organism, while the 

 other half remains 

 as the receptive nu- 

 cleus, or the female 

 pronuclens. The 

 two conjugating nu- 

 clei cannot be distin- 

 guished from those 

 which degenerate, 

 but are apparently 

 only those which lie 

 nearest the bridge 

 joining the two 

 organisms. Mean- 

 while the macro- 

 nucleus undergoes 

 complete degenera- 

 tion, breaking up 



into a number of pieces, which are gradually absorbed by the proto- 

 plasm. The new macronucleus is formed by the enlargement of a 

 daughter-micronucleus derived from the fusion nucleus. Hoyer('99), 

 however, asserts that in Colpidium colpoda it forms by the union of 

 two daughter-nuclei. 



When there are two or more micronuclei in each conjugant, the 

 process is repeated for each of them, although it is not known 

 whether this holds when> as in Stentor, the number reaches sixty or 

 seventy. 



In a few cases (Vorticellidae) the conjugants are of diverse size. 



Fig. 107. Conjugation in Epistyhs umbellana 



Greeff. [GREEFF.J 

 M, macrogamete ; m, microgametes. 



