154 PROTOZOOLOGY 



clones, though its absence results in a marked decrease in vigor. The 

 bi-micronucleate Paramecium bursaria which Woodruff (1931) iso- 

 lated, developed in the course of 7 years of cultivation, unimicronu- 

 cleate and finally amicronucleate forms, in which no marked varia- 

 tion in the vitality of the race was observed. These data indicate that 

 amicronucleates are capable of carrying on vegetative activity and 

 multiplication, but are unable to conjugate or if cell-pairing occurs, 

 the result is abortive, though Chen (1940c) reported conjugation be- 

 tween normal and amicronucleate individuals of P. bursaria (p. 189). 

 Horvath (1950) succeeded in destroying the micronucleus in Kahlia 

 simplex (p. 133) and found the emicronucleates as vigorous as the 

 normal forms, judged by the division rate, but were killed within 15 

 days by proactinomycin, while normal individuals resisted by en- 

 cystment. This worker reasons that the emicronucleates are easily 

 destroyed by unfavorable conditions and, therefore, ciliates without 

 a micronucleus occur rarely in nature. 



Fig. 58. Amitosis of the vegetative nucleus in the trophozoite of 

 Myxosoma catostomi, X2250 (Kudo). 



Other examples of amitosis are found in the vegetative nuclei in 

 the trophozoite of Myxosporidia, as for example, Myxosoma catos- 

 tomi (Fig. 58), Thelohanellus notatus (Debaisieux), etc., in which the 

 endosome divides first, followed by the nuclear constriction. In 

 Streblomastix strix, the compact elongated nucleus was found to 

 undergo a simple division by Kof oid and Swezy. 



Indirect nuclear division. The indirect division which occurs in the 

 protozoan nuclei is of manifold types as compared with the mitosis 

 in the metazoan cell, in which, aside from minor variations, the 

 change is of a uniform pattern. Chatton, Alexeieff and others, have 

 proposed several terms to designate the various types of indirect 

 nuclear division, but no one of these types is sharply defined. For our 

 purpose, mentioning of a few examples will suffice. 



A veritable mitosis was noted by Dobell in the heliozoan Oxnerella 

 maritima (Fig. 59), which possesses an eccentrically situated nucleus 

 containing a large endosome and a central centriole, from which 

 radiate many axopodia (a). The first sign of the nuclear division is 



