104 NUCLEAR DIVISION IN PROTOZOA 



desmose are spindle fibres, at the equator of which chromosomes become 

 arranged. These are formed from the peripheral chromatin granules 

 of the nucleus, and possibly some which have separated from the karyo- 

 some. The chromosomes split to form two plates, which move towards 

 the daughter karyosomes. The centrodesmose and the spindle fibres 

 disappear, while the nuclear membrane is divided by constriction. Two 

 daughter nuclei, each with a larger central karyosome and peripheral 

 chromatin granules, are reconstructed. 



In many cases, as, for instance, in trypanosomes and allied flagellates, 

 in which the nucleus consists of a nuclear membrane containing a large 

 central karyosome, all that can be detected in nuclear division is the 

 elongation of the nuclear membrane, within which the karyosome becomes 

 drawn out and finally dumb-bell-shaped. The narrow intermediate 

 portion may be quite short, or it may be very much drawn out. In 

 either case it finally disappears, leaving two daughter karyosomes. By 

 constriction and division of the nuclear membrane two daughter nuclei 

 are formed (Fig. 156). It is maintained by the advocates of the centriole 

 theory that the narrow intermediate portion in the dumb-bell stage 

 represents a centrodesmose connecting two daughter centrioles which are 

 lodged in the daughter karyosomes. Certain appearances which are some- 

 times seen might lend support to this view. Occasionally, a dividing 

 nucleus may be seen, in which a small granule is situated at each end of 

 the elongated nuclear membrane. These are connected by a fine fibre, 

 at the centre of which the still intact karyosome lies. Such an arrange- 

 ment might be interpreted on the supposition that the centriole within 

 the karyosome has divided prematurely, and that the two daughter 

 centrioles have passed out of the karyosome, which has not yet shown 

 any sign of division. At a later stage the karyosome divides, and the 

 two daughter karyosomes pass to the ends of the nucleus and again enclose 

 the centrioles. Such appearances, however, are unusual, and may be 

 merely accidental arrangements of chromatin granules. Hartmann and 

 Noller (1918) have given another account of the nuclear division in 

 Trypanosoma theileri (Fig. 156). They maintain that the apparently 

 elongated karyosome is really a spindle, at each apex of which is a centriole, 

 and that fine granules of peripheral chromatin form chromosomes which 

 become arranged as an equatorial plate. Mitotic divisions of trypanosome 

 nuclei have been described also by Chagas (1909) and Nieschulz (19226). 



As already intimated, the Protozoan nucleus sometimes contains a body 

 which is entirely devoid of chromatin, and appears to consist of plastin 

 material alone. In nuclear division it may disintegrate and disappear, 

 to be re-formed again in the daughter nuclei. In some cases, however, it 

 divides into two parts, which pass to the poles of the spindle with the 



