SPECIAL MORPHOLOGY OF THE PROTOZOAN NUCLEUS 263 



which play some role in the process of division. The apparent rela- 

 tion of these substances to one another is strikingly similar to the 

 relations between the centrosome and the attraction sphere in Metozoa, 

 but so many points remain obscure that no safe conclusions can be 

 drawn. In one other form, Chilodon cucullulus, there is an intra- 

 nuclear structure which recalls that of Spiroclwna, but nothing is 

 known about its division-phases (Fig. 138, C). 



In all other forms with well-developed "pole-plates" there is a 

 similar mystery regarding the material which enters into their forma- 

 tion. Despite the numerous observations, there is no case on record 



D 



Fig. 139. — Mitotic division in the Infusoria. [From Wilson after R. Hertwig.] 

 A-C. Macronucleus of Spirochona, showing pole-plates. D-H. Successive stages in the divi- 

 sion of the micronucleus of Paramcecium. D. The earliest stage, showing reticulum. G. Follow- 

 ing stage ("sickle-form") with nucleus. E. Chromosomes and pole-plates. F. Late anaphase. 

 H. Final phase. 



in which the origin of the pole-plates has been definitely made out. 

 In all cases, however, it is assumed that the so-called nucleolus or 

 "achromatic body," which is generally present in the resting cell- 

 nucleus, becomes modified in some manner (Hertwig, '95, assumed 

 by the addition of water or other fluid substance) until it is much en- 

 larged, when it forms the pole-plates. This general view is based 

 upon the supposition that, like a centrosome, the achromatic body 

 divides, half going to one pole and half to the other. 



In Sarcodina, as in Infusoria, there is no direct evidence to de- 

 termine the history of the "achromatic" body within the nucleus, 



