Genetics / by Edgar Altenburg.

  • Altenburg, Edgar, 1888-1967.
Date:
[1945]
Catalogue details

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

Credit: Genetics / by Edgar Altenburg. Source: Wellcome Collection.

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    SUMMARY OF CHROMOSOME DISTRIBUTION 9 reduction division as though it involved simply the pairing of two unsplit chromosomes and led directly to the formation of the ma¬ ture reproductive cells, as shown in Fig. 8. This is just a short¬ cut method of representation. The net result of the reduction and equation divisions is that members of a chromosome pair are separated and go to different reproductive cells. The short-cut method gives this result. Why the reduction division should be followed by another divi¬ sion and why the polar bodies should be formed in the female are questions of detail that need not concern us here. The im¬ portant thing for the present is that the mature reproductive cells—the sperm and egg cells— have the reduced number of | chromosomes. Summary of Chromosome Distribution over the Entire Life Cycle.—Figure 9 shows the distribution of the chromo¬ somes from the time the indi¬ vidual begins his existence as a fertilized egg until the time that he is mature (the frog being chosen as an example). In Fiq. s. The reduction division, short- this figure the sperm and egg cut method representation. cells are shown each with three chromosomes (though in the frog the number is actually greater). The fertilized egg therefore has three pairs of chromosomes, or six in all. The fertilized egg divides and forms the first two cells, each with six chromosomes (three pairs). The first two in turn divide and form four cells, each with six chromosomes. The four divide and form eight cells, each again with six chromosomes. As development proceeds,-a mass of cells is formed constituting the young embryo, and each cell still has six chromosomes. As the embryo grows older its cells differentiate and form various tissues, such as the lining of the gut, the muscles, and so on. These continue to have six chromosomes in each of their cells. The young animal also forms its reproductive organs (testes, in the case of the male). These at first contain six chromosomes in each of their cells. But as