FERTILIZATION 613 



In 1898 Dangeard found no reduction taking place during gamete 

 formation in Chlamydomonas and suggested that it occurs during the 

 germination of the egg. Pascher (1916) made no chromosome counts, 

 but presented genetic evidence for zygotic meiosis in Chlamydomonas. 



Hartmann and Nagler (1908) indicated that reduction is zygotic in 

 Sappjnia {Amoeba) diploidea, because three nuclei disintegrate, out of 

 the four that are formed by two zygotic divisions. Diwald (1938) very 

 recently stated that, because he could see no meiosis in the formation of 

 the four gametes of Glenod'mtum lubiniensijorme and because a tetrad 

 of four potential individuals are produced by two divisions of the zygote, 

 reduction occurs in the two zygotic divisions. 



It does not seem justifiable to base an assumption of zygotic meiosis 

 on such indirect and questionable evidence. Genetic evidence must be 

 considered, in the absence of cytological data; but only positive determina- 

 tion of chromosome number and identification of the stage in the cycle 

 in which the number is reduced from diploid to haploid can be accepted 

 as conclusive evidence of this phenomenon. 



Valkanov (1935) has presented fragmentary cytological evidence of 

 zygotic reduction in Monocystella arndti. He shows eleven long chromo- 

 somes in the early divisions of the pseudoconjugants. In the first zygotic 

 division, eleven synaptic pairs condense into short, fat Ys and Xs. He 

 concludes that reduction is zygotic, but his evidence is admittedly incom- 

 plete, as he was unable to follow the subsequent behavior of the chromo- 

 somes. Whether this is truly zygotic meiosis or whether the zygotic pair- 

 ing of chromosomes is a phenomenon similar to that found in Urospora 

 lagidis (see p. 602 above) remains an open question. The odd number 

 of chromosomes lends some support to Valkanov's belief. 



Weschenfelder (1938) has just published what appears to be a clear- 

 cut case of zygotic meiosis in the gregarine Acttnocephalus parvus. In 

 the early nuclear divisions of the pseudoconjugants, four long, rod- 

 shaped, haploid chromosomes are repeatedly observed. Isogametes bud 

 off the mother cell and fertilization occurs as in other gregarines. At the 

 first division of the zygote, eight chromosomes develop from the syn- 

 karyon as four synaptic pairs. These pairs disjoin in the anaphase and 

 four go to each pole, reducing the number of chromosomes to the hap- 

 loid condition again. Subsequent mitoses in the sporoblast reveal the 

 haploid four chromosomes, now globular in shape, appearing in the 

 prophase and passing to each pole in the anaphase. 



