Human chromosomes / E.H.R. Ford.

  • Ford, E. H. R. (Edward Hugh Rawlinson)
Date:
1973
Catalogue details

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

Credit: Human chromosomes / E.H.R. Ford. Source: Wellcome Collection.

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    3. MEIOSIS AND MEIOTIC CHROMOSOMES 67 bodies may be seen in the cell instead of 23. However this separation may be related to the technique of preparation, rather than to an actual separation in vivo. The former interpretation is favoured by the observation that separation is much more frequently seen in air-dried preparations, where the mechanical spreading effect is greater than in squash preparations. (d) First meiotic anaphase. The whole chromosomes move rapidly towards the two poles of the spindle. This is a stage which because of its short duration is not often seen. (e) Telophase. At telophase the haploid chromosome complement is now present in each of the two daughter cells, the secondary spermatocytes', the nuclear membrane reforms, and the reduction division is complete. However this stage is never completed in the same way as mitotic telophase; the daughter cells remain connected by a cytoplasmic bridge, as described earlier, and the chromosomes do not disappear, but remain as dark rods attached to the nuclear membrane until the commencement of the second meiotic division, which probably takes place very shortly after the first is completed. (f) Second meiotic division. The cell now passes, without any effective prophase, into the second meiotic metaphase (Fig. 26). The nuclear membrane dissolves, and the spindle reforms. The 23 chromosomes of the haploid set can be seen aligned on the spindle (or, in the usual preparation, spread upon the slide). Each one displays its two chromatids, joined at the centromere. The morpho¬ logy of the chromatids is rather reminiscent of that of the chromosomes at spermatogonial metaphase; they are coarsely spiralized, and rather fluffy in appearance. At second anaphase the two chromatids constituting each chromosome separate at the centromere and pass to opposite poles; thus at telophase the daughter cells, the spermatids, each contain one complete set of genetic material. This represents one quarter of the material present in the primary spermatocyte from which they originated, and is derived approximately equally from each parent of the individual concerned, and assorted (apart from the X and Y chromosomes, which probably pass intact from one or the other parent) in a random fashion within each chromosome, due to the crossing-over which has taken place. (g) Spermiogenesis. After the second meiotic division is complete the spermatids undergo a complicated series of changes (during which they are infolded in, and presumably nourished by, the Sertoli cells), and become converted into spermatozoa. Since these changes are mainly concerned with the cytoplasmic parts of the cell, and with