This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1896 Excerpt: ... formation of an actual polar body as a small cell segmented off from each of the two conjugating animals soon after their union; but the number of chromosomes was not determined. In the unicellular plants there are indications of a similar process, but the few facts at our command indicate that the reduction may here ...
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1896 Excerpt: ... formation of an actual polar body as a small cell segmented off from each of the two conjugating animals soon after their union; but the number of chromosomes was not determined. In the unicellular plants there are indications of a similar process, but the few facts at our command indicate that the reduction may here take place not before, but after, conjugation of the nuclei. Thus in the dcrmids Closterium and Cosmarium, according to Klebahn (Fig. 99), the nuclei first unite to form a cleavage-nucleus, after which the zygote divides into two. Each of the new nuclei now divides, one of the products persisting as the permanent nucleus, while the other degenerates and disappears. Chmielewski asserts that a similar process occurs in Spirogyra. Although the numerical relations of the chromosomes have not been determined in these cases, it appears probable that the elimination of a nucleus in each cell is a process of reduction occurring after fertilization. F. D1vergent Accounts Of Reduction We can only touch on a few of the accounts of reduction which differ from both the modes already considered. Of these the most interesting are observations which indicate the possibility of, 1. T/ic Formation of Tetrads by Conjugation A considerable number of observers have maintained that reduction may be effected by the union or conjugation of chromosomes that were previously separate. This view agrees in principle with that of Ruckert, Hacker, and vom Rath; for the bivalent chromo 1 Cf. Moore on the spermatogenesis of mammals, p. 201. somes assumed by these authors may be conceived as two conjugated chromosomes. It seems to be confirmed by the observations of Born and Fick on amphibia and those of Riickert on selachians (Pristiurus); for in all these cases the number of...
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