FERTILIZATION 611 



during the process, as in conjugation and endomixis, and the micro- 

 nucleus produces two pronuclei, as in conjugation. The pronuclei fuse 

 autogamously and the synkaryon divides twice to form four nuclei; two 

 are macronuclear Anlagen which separate at the first fission, and two be- 

 come micronuclei. Diller challenges the very existence of endomixis 

 in this species, on the grounds that stages of autogamy and irregular 

 reorganization processes called "hemixis" have been mistaken for endo- 

 mixis. Certainly this challenge must be met by careful reexamination of 

 the studies already made on endomixis (see Woodruff, Chapter XIII; 

 also Sonneborn, Chapter XIV). 



Zygotic Meiosis 



In nearly all animals that have two parents, the two sets of chromo- 

 somes that are contributed to the offspring remain in the nuclei of all 

 cells derived by mitosis from the zygote. This diploid number is charac- 

 teristic of all cells except the gametes, in which case the chromosomes are 

 separated out again by one or two meiotic divisions, giving to each gamete 

 one half the diploid number. This haploid number is found only in 

 the gamete, because the union of gametes reestablishes the diploid condi- 

 tion. 



There is evidence to show that in a few Protozoa, notably among the 

 Telosporidia, reduction in chromosome number occurs in the division 

 of the zygote. This means that the organism lives a haploid existence 

 and only the zygote is diploid. Belaf (1926) argued that since the com- 

 plete reduction process is known only in Aggregata and Karyolysus 

 among Coccidia and in Diplocystis among gregarines, and that in all 

 these forms reduction is zygotic, therefore reduction in all members 

 of the two groups is probably zygotic. He suggested that Mulsow (1911) 

 confused two species in obtaining his results, but the later work of 

 Calkins and Bowling (1926) and of Naville (1927a) on Monocystis 

 has made that conclusion untenable. Belaf further points to the frequent 

 occurrence of odd numbers of chromosomes in these groups as evidence 

 of the haploid condition, which implies zygotic reduction. Odd chromo- 

 some numbers could be explained by postulating a supernumerary or 

 a sex chromosome, or by interpreting each chromosome as in reality a 

 pair in close and prolonged synapsis; but these assumptions are unsat- 

 isfying, in the absence of more adequate evidence. 



