310 



BIOLOGY OF THE PROTOZOA 



Aggregate eberthi and the gregarine Diplocystis schneideri. Dobell 

 (1915) describes 6 chromosomes in the vegetative divisions of 

 Aggregata eberthi, and Jameson (1915 and 1920) describes 3 in DipJo- 

 cystis schneideri (Fig. 56, p. 102, and Fig. 158). These numbers remain 

 constant in both organisms during gametogenesis, the mature 

 gametes have the same numbers while the diploid numbers 12 and 

 6 are present only in the zygotes (Figs. 56 and 158). With the 

 first division of the zygotes the two sets of chromosomes unite 

 in homologous pairs; in Aggregata 1 pair consists of long chromo- 

 somes, 1 pair is very short and 4 pairs are intermediate in length 

 (Fig. 56). The nuclei resulting from this first metagamic division 

 have 6 chromosomes each in Aggregata and 3 each in Diplocystis, and 

 these haploid numbers are retained throughout the vegetative 

 cycles. 



ABC D 



E 



Fig. 158. — Diplocystis schneideri. Zygotic meiosis. A to E, nucleus of the zygote 

 forming 6 chromosomes (the diploid number), and the first metagamic division; F, 

 anaphase of the sixth progamous division preparatory to gamete formation, with 

 3 longitudinally split chromosomes, the haploid number. (After Jameson.) 



The generalization made by Dobell and Jameson to the effect 

 that this method of reduction is probably universal among the 

 Telosporidia is hardly justified by these two cases. Few species 

 indeed have been studied with respect to the reduction of chromo- 

 some number and only one— Monocystis rostrata — by Mulsow (1911), 

 with sufficient care as to cytological detail to be admitted, and 

 here, as stated above, reduction occurs with the final progamous 

 division of the nuclei. Dobell and Jameson would explain this 

 divergent case as due to confusion by Mulsow of stages of two dif- 

 ferent gregarines, one with 8 the other with 4 chromosomes, but 



