368 



BIOLOGY OF THE PROTOZOA 



Macronuclei are generally regarded as ''somatic" nuclei with an 

 important part to play in general metabolism. They disappear by 

 absorption and are replaced by products of micronuclear division at 

 periods of reorganization by "endomixis," or by products of amphi- 

 niiclei after conjugation. Chromosome formation, with a definite 

 number of chromosomes, has been made ovit for a number of species 

 of ciliates, but no definite chromosomes have been described from 

 macronuclei. Evidence is accumulating to indicate that the micro- 

 nucleus is the essential element of the cell in conjugation but other 

 evidence is at hand to show that it is not essential for continued 





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Fig. 159. — Dendrosoma elegans; n, nucleus. (From Calkins after Kent.) 



vegetative life or for reproduction by cell division. Thus amicro- 

 nucleate races of Paramecium, Didinium, Spathidiu7)i , Oxytricha, 

 etc., have been maintained for long periods by Woodruff, Dawson, 

 and others, while Maupas, Calkins and others have shown that the 

 micronucleus may disappear in long-continued cultures of hypo- 

 trichous forms, although the organisms are still able to divide 

 (p. 72). It is evident that different macronuclei represent difl'erent 

 degrees of specialization and that some forms may carry on all 

 processes of asexual activity without a micronucleus and represent 

 transition stages to the opalinids in which there is no nuclear 



