The chromosomes / by M.J.D. White.

  • White, M. J. D. (Michael James Denham), 1910-1983.
Date:
[1961]
Catalogue details

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

Credit: The chromosomes / by M.J.D. White. Source: Wellcome Collection.

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    2 THE CHROMOSOMES it, it becomes indented at the point where the pressure is applied ; if the pressure is released it regains its former shape ; if the pressure is increased it can eventually be punctured.The shape of the nucleus undoubtedly depends in part on the properties of the nuclear membrane. Most nuclei are approxi¬ mately spherical, but many are ovoid. Those of many Vertebrate leucocytes are in the form of a long strand with periodic enlargements (Fig. la) while those of the secretory cells of many insects are very irregularly branched (Fig. 16).In all these] cases of non-spherical nuclei the nuclear surface is very large relative to its volume, and it has been assumed that this is connected with the process of secretion ; but many secretory cells (such as the salivary gland cells of Diptera) have approximately spherical nuclei. Some unusual nuclei do not consist of a single body at all, but of a number of separate vesicles, each con¬ taining a single chromosome and a certain amount of nuclear sap inside a separate membrane (Fig. Ic). In one case the sex chromosome is enclosed in a separate membrane from the main nucleus (Fig. \d). The nuclear sap is usually a clear fluid ; its vis¬ cosity has been determined in one case to be about twice that of the water,and this is probably typicaluDf most nuclei ; in some, however, the nuclear sap may be a solid gel. The amount of nuclear sap relative to the volume of the chromosomes varies enormously from one type of nucleus to another. Thus in the micronucleus of Ciliates and in the sperm- head nuclei of many animals there is practically no nuclear sap ; on the other hand, the total volume of the young oocyte nuclei of birds (diameter up to 100 /л) may be 200,000 times that of the chromosomes at metaphase. The chromosomes may be, and usually are, in¬ visible in living nuclei during the resting stage.2^- 60, 100 Хд some plant nuclei, however, and also in a few animal nuclei, fine threads can be seen in the