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

  • Michael James Denham White
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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    4 THE CHROMOSOMES certain, since in some cases they become visible at the beginning of one mitosis in the same position as they occupied at the end of the preceding division. Also, in some cases special portions of chromosomes persist in a condensed state throughout the resting stage {'prochromosomes).^'^ It is now becoming increasingly clear that no idea of the structure of the resting nucleus can be obtained from studying fixed preparations. The usual text¬ book figure of a ' network of linen threads with granules of chromatin at the points of intersection ' is meaningless save as a description of a gross artefact which bears only the most remote relation to the living structure. We must resign ourselves to the fact that the resting nucleus and its chromosomes, due probably to their high water content, are unfix- able. It should be emphasized that this only applies to resting nuclei—there is every reason to believe that fixed preparations of nuclei in mitosis present a very accurate picture of what is taking place in the living cell (see next chapter). Most cells in the body of an adult human being have undergone about 50 mitoses since the fertilized egg (i.e. an adult man consists of about 10^^ cells, making allowance for the erythrocytes, sperms and other cells which are constantly being destroyed and replaced). In the case of insects the adult cells have undergone only about 20-30 divisions, and in the case of the Nematoda and Rotifera still fewer. Be¬ tween each of these divisions a resting stage has intervened. These resting stages are, however, of very uneven duration ; on the one hand two divisions may follow on one another without being separated by any resting stage at all (the end of one division passing directly into the beginning of the next)—or on the other hand the resting stage may last for years as in many adult tissues of vertebrates. Many adult cells may be said to have entered a permanent resting * stage, since they will never divide again. In most