CILIOPHORA 



453 



cession to form eight nuclei, of which four enlarge to become 

 macronuclei, one remains as a micronucleus and three disin- 

 tegrate and disappear. The one micronucleus then divides by 

 mitosis and the cell divides to form two paramaecia, each contain- 

 ing one micronucleus and two ^ 

 macronuclei. The next division 

 gives rise to cells containing the 

 normal number of nuclei, one 

 micronucleus and one macro- 

 nucleus. 



The paramaecia are lagre sapro- 

 phytic organisms, easily kept under 

 cultivation in the laboratory, and 

 they have been very extensively 

 studied. Many conceptions 

 founded upon these studies are 

 considered to have a broad bear- 

 ing upon the physiology of all 

 living cells. For example Jen- 

 nings^ has found that conju- 

 gation serves two purposes, (i) to 

 provide chemical stimulation of 

 cell division and (2) to insure va- 

 riety in the descendants. The variety in the descendants is a 

 result of the exchange of nuclear material. Calkins^ has dis- 

 covered a specialization of function in paramaecium in respect 

 to conjugation and concludes that in some of the descendants 

 of an px-conjugant the ability to conjugate is in abeyance, 

 thus suggesting a resemblance to the somatic cells of a metazoon, 

 . while other descendants retain this function and are therefore 

 analogous to the germ cells of a metazoon. 



Three other species of Paramecium are recognized, namely, 

 P aurelia. P. bursaria and P. putrinum. 



' Harvey Lectures, 1911-12, pp. 256-276. 



"iProc. Soc. Exp. Biol, and Med., 1913, Vol. X„pp. 65-67. 



Fig. 217. — Paramaecia drawn at 

 the same magnification. A, Para- 

 mcecium caudatum. B. Paramaciuni 

 putrinum. (From Doflein after Sche- 

 wiakoff). 



