MITOSIS 95 



divisions take place by mitosis. When the nucleus of a gamete in the 

 secondary cyst is about to divide for the first reduction division, there 

 appears at one side of the nucleus an area of clear cytoplasm towards 

 which the linin network of the nucleus with its chromatin granules is 

 drawn (Fig. 54). Into this clear cytoplasm some of the chromatin granules 

 of the nucleus are attracted, and by their aggregation give rise to the 

 centrosome. It is possible the centrosome was already present, either 

 in the nucleus or outside it, and that the commencement of its activities 

 results in the concentration of the nuclear elements at this pole of the 

 nucleus, and even the escape of some of the chromatin into the cytoplasm. 

 Whether Hert wig's account of the origin of the centrosome is correct or 

 not, when it becomes apparent it is situated at some distance from the 

 nuclear membrane, and is surrounded by radiations, the bulk of which 

 are directed towards the nuclear membrane. Division of the centrosome 

 takes place, and one of the resulting pair takes up a position at the oppo- 

 site pole of the nucleus. There are now two asters between which spindle 

 fibres appear. The nucleus occupies a position between the two centro- 

 somes, and the spindle fibres extend through the nuclear membrane and 

 the substance of the nucleus, so that there is both an extranuclear and 

 an intranuclear portion of the spindle. The chromatin granules of the 

 nucleus now form a series of chromosomes which become arranged in the 

 form of a plate across the equator of the spindle within the nuclear 

 membrane. Each chromosome divides, and there result two daughter 

 plates which, just behind the ends of the elongating nuclear membrane, 

 move towards the centrosomes. The nuclear membrane, which has 

 divided, now closes round the chromosomes, which gradually disintegrate, 

 so that daughter nuclei are formed. One of the nuclei now degenerates. 

 As already remarked, the nuclei of the two individuals in the secondary 

 cysts undergo two such divisions, the description just given applying to 

 the first of these. The second division is of a similar type, and again one 

 of the daughter nuclei degenerates. Richard Hertwig regards both these 

 divisions as reduction divisions, though he believed that the chromosomes 

 actually divide in each instance. It seems reasonable to suppose, from 

 wdiat is now known to occur in other Protozoa, that in one of the two 

 divisions splitting of the chromosomes does not take place, but that they 

 separate into two groups, so that the number of chromosomes in the 

 daughter or final gamete nucleus is halved. This is all the more probable 

 since, in Actinophnjs sol, an allied form which has a similar syngamic 

 process, Bela' (19216, 1923) has noted that in the first of the divisions 

 the chromosome number of forty-four is reduced to twenty-two (Fig. 50). 

 To return to AcfinnspluBnuni eichhorni, the many nuclei which an adult 

 contains become the nuclei of the daughter individuals which form the 



