110 



THE PROTOZOA 



formation of a definite equatorial plate which splits into two 

 daughter-plates. The most striking and salient feature of this type 

 of mitosis is furnished by the relatively huge " polar masses," con- 

 sisting of the daughter-karyosomes with their contained centrioles. 

 In the division of the nucleus of Arcella (Fig. 57), however, the 

 karyosome first breaks up into fine grains of chromatin, from which 

 the polar masses and the equatorial plate are formed. The karyo- 

 some, as has been pointed out in the previous chapter, consists of 

 three distinct elemente namely, plastin, chromatin, and centriole 



Fio. 57. Nuclear division in Arcella vulgaris: karyo kinesis of 

 one' of the two principal nuclei. A, Spireme-stage, resulting 

 from disruption of, the karyosome ; B D, formation of an 

 equatorial plate of minute chromosomes (?) which split ; 

 E, antiphase ; F, the two daughter-nuclei shortly after 

 division. After Swarczewsky (101), magnified 2,250. 



each independent of, and separable from, the 

 others. In proportion as the karyosome loses its 

 plastin and chromatin elements, and becomes reduced 

 to the centriole alone, so the primitive promitosis 

 will approach more and more to the type of an 

 ordinary mitosis. Such a reduction of the karyo- 

 some could take place during the mitosis if, as 

 happens frequently, the whole of the chromatin 

 p contained in the karyosome passed out to join the 



peripheral chromatin in forming the chromosomes, 

 the plastin-substance at the same time furnishing the required 

 ground-substance of the chromosomes (Fig. 58). On the other 

 hand, the karyosome may disappear from the- resting nucleus 

 also ; Chatton (49) has brought together a number of instances 

 of nuclei showing a gradual reduction of the karyosome in 

 different species, and the evolution of a granular type of nucleus 

 in which the chromatin is scattered through the achromatinic 

 framework, leaving the centriole free or but slightly encumbered 

 by other elements in the nuclear cavity. When a nucleus of this 

 type divides by mitosis, a most typical and perfect karyokinetic 



