THE INFUSORIA 



191 



as a rule, by amitosis in the macronuclei. The latter is the simpler ; 

 in many spherical or elliptical nuclei the structure merely draws out 

 and segments into two equal parts. It is more or less complicated, 

 however, in different macronuclei, until well-developed mitosis replaces 

 simple division {e.g. SpirocJiona). There is reason to regard the sim- 

 ple division of the larger nuclei as the mere degeneration of mitosis, 

 by a process in which the various stages have gradually disappeared 

 until only the preliminary stages of such division are to be found. 

 These preliminary stages are seen in the transformation of the reticu- 

 lum of chromatin into thread-like masses, which recall the spireme of 



B 



D 



Fig. 105. — Mitotic division of the nuclei of Spirochona and Paramecium, [R. HERTWIG.] 

 A-C. Macronucleus of Spirochona with well-developed pole plates. D-H. Different stages in 

 division of the micronucleus of Paramecium. 



higher cells ; the threads are then divided across into equal parts. In 

 Spirochona, however, the process of division is strikingly similar to 

 mitosis in Metazoa (Fig. 105, B, C). 



Division of the micronuclei is accompanied by the formation of 

 polar attraction-spheres, and by the rearrangement of chromatin into 

 chromosomes. Before division, the micronucleus swells to nearly 

 twice its size during resting stages (Hertwig, 'yj\ while the granu- 

 lar chromatin begins to collect in lines — at first equally thick, but 

 later concentrated in the equatorial region (Fig. 105, D-H). Divi- 

 sion then takes place through the centre. 



