MEIOSIS 109 



Dobell and Jameson (1915) have given a description of a reduction division 

 which differs from that of Muslow. According to these observers, during 

 all the division stages of the nuclei, including the last division which gives 

 rise to the gamete nuclei, there are three chromosomes which divide to 

 form the chromosomes of the daughter nuclei (Fig. 65). The nuclei of 

 the gametes thus have three chromosomes, as do the nuclei of the preceding 

 stages. When the gametes unite and their nuclei fuse, the zygote nucleus 

 has six, or double the number of chromosomes found at other stages. 

 The zygote nucleus now proceeds to division, and it is in this division that 

 the reduction occurs, three of the six chromosomes passing to each daughter 

 nucleus. At all subsequent division stages of the nuclei the three chromo- 

 somes are divided. In Muslow's account of Monocystis rostrata it was 





? ^ 





1 2 



Fig. 64. — Last Nuclear Division in One of Two Associated Gregarines, 

 Monocystis rostrata, to show the Reduction of the Chromosome Number 

 FROM Eight to Four in the Gamete Nuclei ( x 5,000). (After Muslow, 1911.) 



1. Eight chromosomes in nucleus. 2. Eight chromosomes arranging themselves in pairs. 



.•{. Separation of the individual chromosomes of each pair. 



4. Four chromosomes moving to each pole of the spindle to form the gamete nuclei. 



during the last nuclear division in the production of gamete nuclei that 

 the number of chromosomes was halved, whereas in Dobell and Jameson's 

 account of Diplocystis schneideri the reduction does not occur at this 

 stage, but at the first division after the zygote nucleus has been formed. 

 According to Muslow, the haploid number of chromosomes of Monocystis 

 rostrata is four, and occurs in the gametes, while all other nuclei have the 

 diploid number of eight chromosomes; on the other hand Dobell and 

 Jameson in Diplocystis schneideri find that the diploid number six 

 occurs only in the zygote, all other stages showing the haploid number 

 three. The latter observers have noted the same condition in the case 

 of the coccidium Aggregata eberthi (Fig. 66). In this parasite, during 

 schizogony nuclear divisions occur in which six chromosomes appear in 

 the equatorial plate, and they all divide so that the daughter nuclei have 



