92 NUCLEAR DIVISION IN PROTOZOA 



division of Acanthocyslis aculeata, one of the Heliozoa, as described by 

 Scliaudinn (189G6), (Fig. 51). In the ordinary individual the centre of 

 the body is occupied by a granule, from which radiate the axial fibres 

 supporting the fine pseudopodia. The nucleus, which has a membrane 

 and large central karyosome, lies at one side of the central granule. When 

 division is to take place, the nucleus increases in size and the karyosome 

 becomes loculated, broken into a number of separate parts, and finally 

 disintegrated as minute granules which arrange themselves in the form 

 of a spireme or coiled thread. Meanwhile the supporting fibres of the 

 pseudopodia have disappeared, while radiating fibres develop in the 

 cytoplasm in connection w^ith the central granule, which, on account of 

 the part it plays in nuclear division, must be regarded as the centrosome. 

 The latter structure divides, and as the two daughter centrosomes separate 

 a spindle is formed between them, while radiating fibres form two asters. 

 The nucleus, within which the spireme has segmented into a number of 

 separate parts, now moves to the equator of the spindle. The nuclear 

 membrane disappears, and a number of small chromosomes take up a posi- 

 tion on the spindle as an equatorial plate. The individual chromosomes 

 divide, and there are formed two daughter plates which move towards 

 opposite poles of the spindle. At this stage the body of the Heliozoon, 

 which has become elongated, begins to show a constriction around its 

 centre. The spindle is finally divided at its centre, and the daughter 

 chromosomes of each plate become transformed into a karyosome, while 

 a new nuclear membrane is developed. The centrosome remains as the 

 central granule of the daughter individual which has been formed, and 

 new axial fibres are developed. In this division, practically all the stages 

 of mitosis as seen in the Metazoan cell occur. 



Typical examples of mitosis occur also in the case of gregarines, the 

 nuclei of which divide repeatedly to form the gamete nuclei. Muslow 

 (1911) has described the process as it occurs in Monocystis rostrata, one 

 of the species of Monocystis which inhabit the vesicula seminalis of the 

 earth-worm. The resting nucleus consists of a nuclear membrane and 

 large central karyosome. When the first nuclear division is to take place 

 after two gregarines have become encysted together in the gametocyst, 

 the large karyosome breaks up, while a long twisted thread of chromatin 

 granules appears at one side of the nucleus (spireme stage). Meanwhile, 

 from two small areas on the surface of the nuclear membrane, radiations 

 appear in the cytoplasm to form the two asters. Between these, spindle 

 fibres develop, and with the disappearance of the nuclear membrane the 

 chromatin thread becomes segmented into eight looped chromosomes, 

 which arrange themselves at the equator of the spindle. Each chromo- 

 some becomes divided longitudinally, and the two groups of eight daughter 



