NUCLEAR DIVISION IN PROTOZOA. 



225 



while Hertwig's appears to be somewhat better founded. Both 

 observers find that the resting nucleus consists of a basis of 

 achromatin in the form of a network, which thickens to form 

 a peripheral zone, while chromatin granules and plastin mate- 

 rial lie within its meshes. When the nucleus is preparing for 

 division, two heaps of homogeneous cytoplasm appear at oppo- 

 site sides against the nuclear membrane (cytoplasmic Kegel). 

 The nucleus becomes pressed together between these two 

 masses, until it forms a lens-shape body, while within the 

 nucleus, homogeneous masses — the pole plates — appear at 

 each pole. Between the two pole plates the nuclear network 

 becomes arranged in spindle fibers, upon which the chromo- 

 somes gather, at first along the entire course of each fiber, 

 but later concentrated as distinct chromosomes in a central 

 nuclear plate. Hertwig definitely states that the pole plates 

 are formed from the same substance as the connecting fibers, 

 vi::., the network within the nucleus, while the curious cyto- 

 plasmic and extranuclear heaps against the nuclear membrane 

 are considered by both Hertwig and Brauer to come from within 

 the nucleus. The substance of both pole plates and sphere, 

 therefore, they think to be the same. In the maturation 

 mitoses a complication is brought about by the presence of 

 centrosomes. The formation of centrosomes is first indicated 

 by protoplasmic radiations formed from the homogeneous extra- 

 nuclear substance (Hertwig). The chromatin, meantime, collects 

 at the pole of the nucleus turned towards this external mass, 

 and from it a small portion is budded off to become the cen- 

 trosome. This chromatin bud rapidly enlarges, assumes a 

 spongy structure, and two granules appear within it (Hertwig 

 calls these the "centrioles" of Boveri). After these are formed, 

 the spongy mass (Hertwig calls it archoplasm) disappears, leaving 

 the centrioles isolated within the cytoplasmic heap. Hertwig 

 did not make out the origin of the other centrosome. Here, 

 as well as in the vegetative mitoses, the nuclear membrane 

 remains intact during the entire process ; pole plates, less 

 conspicuous than in the ordinary mitoses, are developed, and 

 spindle fibers are formed within the nucleus. 



This somewhat bizarre account of the origin of centrosomes 



