MORPHOLOGY AND TAXONOMY OF THE INFUSORIA 477 



there are two, in P. multimicronucleata there are many and two are 

 characteristic of the Oxytrichidae, etc. The number of micronuclei 

 runs up to eighty or ninety in Stentor and the number is intermediate 

 in several other genera. 



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- 

 nuclei after conjugation. Chromosome formation, with a definite 

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



W &,; M& 



Htm M wm^mSk i\f%A: 





j m 



'W n 



Fig. 196. — Dendrosoma elegant; n, nucleus. (From Calkins after Kent.) 



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 

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

 nucleate races of Paramecium, Didinium, Spathidium, 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. 256). It is evident that different macronuclei represent different 



