54 



PROTOZOAN NUCLEUS 



to suppose that the nucleus and the kinetoplast of a flagellate represent 

 two nuclei, as do the micronucleus and the macronucleus of a ciliate. 

 Such an assumption has been made by Hartmann (1907) and others, 

 who regard the kinetoplasts as true nuclei, and the flagellates which possess 

 them as constituting a special group of the Mastigophora, the Binucleata. 

 As pointed out by Alexeieff (19176), there is no ground for this assumption, 

 and in order to avoid confusion he proposed the name kinetoplast in place 

 of kinetonucleus and other terms which implied a nuclear nature. It is 



safer to regard the kinetoplast as a 

 distinct structure concerned with the 

 activities of the flagellum, even though 

 it divides when cell division occurs, 

 and possibly may have originated from 

 the nucleus in the first place. It might 

 equally be argued that though the 

 macronucleus of a ciliate has originated 

 from the micronucleus, and though it 

 multiplies by division during reproduc- 

 tion, it has ceased to be a nucleus in the 

 true meaning of the term, and has be- 

 come modified to serve some other pur- 

 pose, possibly in connection with the 

 development of large numbers of cilia - 

 It is worthy of note that the macro- 

 nvicleus does not divide like a true 

 nucleus, which in most cases, at least, 

 shows some indication of mitosis. 

 Against this view, however, can be 

 raised the argument that there occur 

 certain races of ciliates which possess 

 no micronuclei, though other races of 

 the same species have both micro- and 

 (p. :^5), Dawson (1919) discovered an 

 amicronucleate race of Oxytricha hyiiienostoma, a ciliate which normally 

 possesses both nuclei. The ciliate was kept in culture for several years, and 

 though possessing only a macronucleus, it reproduced regularly by fission. 

 ENDOMIXIS. — As already remarked, the macronucleus of a ciliate 

 degenerates during conjugation, and a new macronucleus is developed 

 from one of the products of division of the micronucleus. This re- 

 placement of the macronucleus from the micronucleus may occur at 

 times other than during conjugation. Wlien Paramecium aurelia 

 reproduces repeatedly by simple division over long periods at certain 



\^- 





Fig. 33. — Bodo caadatus Coprozoic 

 IN Human F.^ces ( x ca. 1.500). 

 (Original.) 



I and 2. Forms showmg two blepharo- 



plasts with associated parabasal. 

 3. Small individual. 4. Encysted form. 



macronuclei. As noted above 



