Archive image from page 103 of Cytology, with special reference to. Cytology, with special reference to the metazoan nucleus cytologywithspec00agar Year: 1920 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 str
Archive image from page 103 of Cytology, with special reference to. Cytology, with special reference to the metazoan nucleus cytologywithspec00agar Year: 1920 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 (vSchleip, 1909) there occurs a synizesis from which in the fonner 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 duphcity 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 is B C Fig. 41. A, parthenogenetic oocyte of Daphnia piilex during the growth stage stagC, hoWCVCr, (after Kuhn, , 1908); B, C, chromosomes of the maturation divisions j j j of two Ostracods. B, N'otodromas monacha (sexual) ; C, Cypris fuscala both prCCCdcd aUQ (parthenogenetic) (after Schleip, , 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 denjdng altogether the connection of the ordinary meiotic phenomena (zygotene stage, etc.) with the
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