130 PROTOZOOLOGY 



behave in the same way as do those which arise from microniiclei. 

 Thus the macronucleus in this ciHate appears to be a compound 

 structure with its 40 or more component parts, each containing all 

 that is needed for development into a complete macronucleus. From 

 these observations, Sonneborn concludes that the macronucleus in 

 P. aurelia appears to undergo amitosis, since it is a compound nu- 

 cleus composed of 40 or more "sub-nuclei" and since at fission all 

 that is necessary to bring about genetically equivalent functional 

 macronuclei is to segregate these multiple subnuclei into two 

 random groups. 



a b c d e 



Fig. 57. Amitosis of the vegetative nucleus in the trophozoite of 

 Myxosoma catostonii, X2250 (Kudo). 



Other examples of amitosis are found in the vegetative nuclei in 

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

 tomi (Fig. 57), Thelohanellus notatus (Debaisieux), etc., in which the 

 endosome divides first, followed by the nuclear constriction. In 

 Strehlomastix 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 examples will suffice. 



A veritable mitosis was noted by Dobell in the heliozoan Oxnerella 

 maritima (Fig. 58), 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 

 the slight enlargement, and migration toward the centriole, of the 

 nucleus (6). The centriole first divides into two (c, d) and the nucleus 

 becomes located between the two centrioles (e). Presently spindle 

 fibers are formed and the nuclear membrane disappears (/, g) . After 

 passing through an equatorial-plate stage, the two groups of 24 

 chromosomes move toward the opposite poles {g-4). As the spindle 

 fibers become indistinct, radiation around the centrioles becomes 



