VII 



MITOSIS IN PROTISTA 



205 



the chromatin granules in linear series had the same function as postu- 

 lated for the chromosomes of a Metazoan mitosis, namely, to effect their 

 accurate division and partition among the daughter nuclei. 



In some Protista very well-developed mitosis, closely resembling 

 that in the Metazoa and Metaphyta, is found (Fig. 85). 



In the Coccidian, Aggregata eberthi (Dobell and Jameson, 1915) the 

 nuclei of the primary gametocytes ( $ and ? ) show in mitosis six 

 chromosomes of very different sizes (labelled, from largest to smallest, 

 a-f in Fig. 86). The macrogametocyte is transformed into the macro- 

 gamete without any reduction of 

 chromosomes, the macrogamete 

 having therefore the same series 

 of six chromosomes. The pri- 

 mary microgametocyte nucleus 

 undergoes repeated division to 

 form the microgamete nuclei, the 

 same series of six chromosomes 

 appearing throughout, though 

 becoming greatly reduced in size. 

 Both gametes have therefore, like 

 the gametocytes, six chromo- 

 somes. Syngamy results in a 

 zygote with twelve chromosomes 

 which can be sorted out into 

 pairs as in a typical Metazoan 

 diploid nucleus. 



In the metaphase of the first 



,. . . _ Mitosis of the gametocyte nucleus of Monocystis. 



division Of the ZygOte nUCleUS the (After Brasil, Arch. Zool. Exp. GY., 1905.) A, B, pro- 

 phase ; C, anaphase. 



homologous chromosomes become 



united into bivalents, the constituents separating at anaphase. This 

 division therefore is a reduction division and the daughter nuclei 

 have only six chromosomes. This number is retained throughout 

 all the subsequent nuclear divisions of the life cycle, which include 

 spore formation, the asexual multiplication of the schizont, and the 

 gametocyte divisions again. Thus in this animal the relation be- 

 tween the duration of the haploid and diploid phases is the reverse of 

 what obtains in the Metazoa, the nuclei being haploid throughout all 

 the life cycle except in one cell generation the zygote cell while 

 meiosis takes place, not at gametogenesis, but at the first division of 

 the zygote nucleus, which is comparable to the first cleavage mitosis 

 of the Metazoan egg. Many plants also exhibit a much longer duration 

 of the haploid generation relatively to that of the diploid part of the life 



