612 FERTILIZATION 



In their preliminary report, Dobell and Jameson (1915) gave the 

 main features of their later detailed descriptions of the life cycles of 

 the gregarine Diplocystis schneideri (Jameson, 1920) and the coccidian 

 Aggregatd ehertht (Dobell, 1925 ) . In both cases meiotic reduction occurs 

 in the first division of the zygote, and the organism lives all the rest of 

 its life as a haploid animal. 



In Diplocystis the nucleus of each pseudoconjugant divides many 

 times, showing the haploid three chromosomes at each division. The 

 dividing nuclei migrate to the periphery and eventually form club-shaped 

 gametes, which pinch off from the gametocyte and fuse with those of 

 the other pseudoconjugant. After a synaptic clumping of bead-like 

 chromatin threads, six chromosomes appear in the prophase of the zygo- 

 tic division and three go to each pole. The diploid number of chromo- 

 somes in the zygote is therefore reduced at the first amphinuclear divi- 

 sion to the haploid three, a number which is also observed in the later 

 divisions of the sporoblast. 



Aggregatd eberthi, like other Coccidia, has well differentiated male 

 and female gametes. The female gametocyte is transformed bodily into 

 a macrogamete after a complicated nuclear reorganization, which does 

 not, however, involve reduction, though a spindle is formed and the 

 six haploid chromosomes are seen. The nucleus of the male gametocyte 

 divides by a complicated method, showing six chromosomes at each 

 division. When the small flagellated microgamete fertilizes the macro- 

 gamete, a fertilization membrane appears. The diploid number of twelve 

 chromosomes appears on the spindle of the first zygotic division. Pairing 

 of homologous chromosomes follows, and the bivalent chromosomes be- 

 come closely applied to each other. They later disjoin and six go to 

 each pole, thereby reducing the number to the haploid condition. All 

 other divisions of the nuclei are mitotic and six chromosomes appear and 

 are divided at each mitosis. 



It is possible that in both Aggregatd and Diplocystis disjunction does 

 not occur in the metaphase of the first zygotic division, and that, instead 

 of this the bivalent chromosomes divide equationally, with six bivalents 

 going to each pole. If at each subsequent mitosis the bivalent chromo- 

 somes divided until just before gamete formation and then disjoined, 

 then reduction would be gametic instead of zygotic. However, the evi- 

 dence seems conclusive enough to convince most biologists that in these 

 two cases meiosis is truly zygotic. 



