MEIOSIS 113 



division the six chromosomes divide, so that each daughter nucleus has 

 six chromosomes (Fig. 66, L and M). Eventually, a large number of 

 nuclei are formed. These arrange themselves on the surface of the cyto- 

 plasm, which segments into a number of sporoblasts. 



An exactly comparable process has been described by Reichenow 

 (192 1) in the case of haemogregarines of the genus Karyolysus (Fig. 457). 

 Here the haploid number of chromosomes is four, and these occur in nuclei 

 of all stages except those of the zygotes, which have the diploid number 

 of eight. When the zygote nucleus divides, four closely united pairs of 

 chromosomes occur at the equator of the spindle. One chromosome of 

 each pair then passes towards the pole of the spindle, so that the resulting 

 daughter nuclei have again only four (see p. 1098). These accounts agree 

 in that the reduction division occurs at the division of the zygote nucleus, 

 and not, as Muslow maintains, in the last division which gives rise to 

 the gamete nuclei. It seems highly improbable that Monocystis rostrata 

 would differ from other gregarines or coccidia in this respect, and Dobell 

 and Jameson have suggested that possibly Muslow was dealing with a 

 mixed infection of two gregarines, one of which has a chromosome number 

 of four and the other of eight, and that what he considered to be the 

 reduction division of the form with eight chromosomes was in reality the 

 ordinary division of the form with four chromosomes. 



The nuclear division during the vegetative reproduction by binary 

 fission, the formation of gametes, and their maturation in the Heliozoon 

 Actinophrys sol has been the subject of detailed study by Belaf (1923), 

 as mentioned above. The organism reproduces by simple division. 

 Finally, encystment occurs and the uninucleated individual within the 

 cyst divides to form two gametes (Fig. 50). The nucleus of each gamete 

 divides and one of these degenerates. The remaining nucleus then 

 divides, and one of the resulting nuclei degenerates. There have thus 

 been two maturation divisions of the gamete nuclei. Conjugation of 

 gametes then occurs.. During vegetative reproduction the nucleus 

 divides without centrosomes by mitosis, while retaining its nuclear mem- 

 brane. When the chromosomes, which number forty-four, first appear 

 during nuclear division they are thread-like, but as the equatorial plate 

 stage is reached they become much shortened, and finally roughly spheri- 

 cal, in which condition they divide to form daughter chromosomes. 

 When the encysted individual divides to form the two gametes, the nuclear 

 division is of the same type as that occurring during the ordinary vegeta- 

 tive reproduction. The forty-four long chromosomes become arranged 

 in twenty-two pairs, the members of each pair being closely applied to 

 one another. Finally, when the equatorial plate stage is reached, there 

 are present at the equator of the spindle twenty-two pairs of more or 



