88 



CYTOLOGY 



CHAP. 



centres in the details of the meiosis, as we may call it, though it does 

 not result in a reduction of the chromosome number. 



A puzzling feature about the changes undergone by the nucleus in 

 preparation for the single maturation division is their extraordinary 

 similarity to the typical phases of a true meiosis resulting in chromosome 

 reduction. The definitive chromosomes of the single maturation division 

 are also strikingly similar to those found in true meiosis, though the 

 former are univalent and the latter bivalent (Figs. 41, 42). 



Thus, in both sexual and parthenogenetic Ostracods (Schleip, 1909) 

 there occurs a synizesis from which in the former the haploid, and in 

 the latter the diploid, number of chromosomes emerges. These chromo- 

 somes are remarkably alike in appearance in the two types of eggs, being 

 conspicuously double in both (Fig. 41). In the one case, however, the 

 duplicity is due to bivalency, in the other to the prophase division of 



univalents. Kuhn 

 (1908) found in par- 

 thenogenetic Clado- 

 cera a stage with 

 conspicuous dupli- 

 city of chromatin 

 threads, strongly 

 suggesting syndesis 

 (Fig. 41). This 

 however, is 







<•. 



m/«» 



B 



Fig. 41. 



A, parthenogenetic oocyte of Daphnia pulex during the growth stage stagC, 

 (after Kuhn, A Z., 1908); B, C, chromosomes of the maturation divisions j j j 



of two Ostracods. B, Notodromas monacha (sexual) ; C, Cypris tuscata both preceded and 

 (parthenogenetic) (after Schleip, A.Z., 1909). 



followed by one m 

 which the chromosomes are obscured by their relation to the nucleolus, 

 thus making correct interpretation difficult. 



The similarity between the meiotic processes of sexual and partheno- 

 genetic species has indeed been cited by some cytologists as reason for 

 denying altogether the connection of the ordinary meiotic phenomena 

 (zygotene stage, etc.) with the reduction of the chromosomes. This is 

 undoubtedly going to an unjustifiable length, for (i) none of the animals 

 which exhibit parthenogenesis are really favourable objects for cy to- 

 logical study. They cannot be compared in this respect with, say, 

 Tomopteris, Lepidosiren, or the Amphibia, and consequently the details 

 cannot be said to be satisfactorily known ; and (2) the fact that the 

 diploid chromosome number appears in late prophase and metaphase 

 in parthenogenetic eggs is no proof that syndesis did not take place in 

 earlier prophase. We ha^'e only to remember the complete dissociation 

 of the ex-syndetic homologous chromosomes in the spermatogenesis of 

 Lepidosiren and in many oogeneses to agree with this proposition. It 

 is not impossible that, in the examples of parthenogenesis now under 



