The genetics of Paramecium aurelia / [Geoffrey Beale].

  • Beale, Geoffrey, 1913-
Date:
1954
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: The genetics of Paramecium aurelia / [Geoffrey Beale]. Source: Wellcome Collection.

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    The Material distributed, without multiplying, to the products of successive divisions of the ex-conjugants, and may soon disintegrate com¬ pletely. Sonneborn (i947¿) mentions that variability in dura¬ tion of persistence of macronuclear fragments is probably connected with differences in nutritive conditions. The two micronuclei in each conjugant undergo a complex series of processes during conjugation. Each of them increases considerably in size, and then passes through two divisions, the first lasting two-thirds of the time the animals are joined to¬ gether, the second following immediately after without an inter¬ vening resting stage. A characteristic feature of the first of these divisions is the formation of crescent-shaped structures which may be as long as 20/1. Subsequently there is an appearance of many minute chromosomes arranged on a metaphase plate. From studies with other ciliates having larger chromosomes, and from the general course of events, it is reasonable to infer that the first two micronuclear divisions constitute meiosis, though the cytological details of the process have not been clearly observed in P. aurelia. Thus eight, presumably haploid, nuclei are formed in each conjugant. It is usually considered that seven of these eight nuclei disappear (though in some accounts it is stated that some may persist for a while). The remaining haploid nucleus is to be found in a peculiar structure near the border between the mates, in the region of the old mouths. This structure, called the paroral cone, has been mentioned above (p. 23), but not defined. Named by Diller (1936), it is a protuberance formed in the mouth region of each conjugating animal, and juts into the body of the partner. As viewed from the side, the two cones overlap each other. According to Sonneborn (1954^), one function of the paroral cone is to protect the contained nucleus from the destructive condition of the cytoplasm, which develops at a certain stage of conjugation, and results in the disintegration of all micronuclei situated else¬ where than in the paroral cone. The 'protected' nucleus passes through a third, presumably mitotic, division in the paroral cone, and as a result, two gamete nuclei are formed in each conjugant. One—the 'stationary' or ' female '—gamete nucleus passes back into the interior of the 25
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