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Contributions from the Zoological Laboratory of the University of Pennsylvania Volume 20
Contributions from the Zoological Laboratory of the University of Pennsylvania Volume 20
Contributions from the Zoological Laboratory of the University of Pennsylvania Volume 20

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    This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1918 Excerpt: ...have been found in which there appeared to be a constriction but this interpretation was always open to considerable doubt. In figure 36, to the left of the number is one of the best cases of a thin area in a chromosome that I have found. One of the chromosomes in figure 85 also shows constrictions. When a large number of chromosomes are concerned, it becomes a matter of considerable difficulty to determine whether a chromosome has been found showing a constriction or whether this dent is really due to the overlapping or approximation of two chromosomes. Although it has been impossible actually to demonstrate the possibility of fragmentation through the discovery of constrictions in the chromosomes, there are, however, indirect methods of proving that this process is going on in the pig. If the chromosomes are breaking up we would expect to find, as in the case of Oenothera scintillans, that those cells showing a high degree of fragmentation would give ocular evidence of the fact by possessing visibly shorter chromosomes. Those cells which possess only a few more than the spermatogonial number of chromosomes should appear like the germ cells, while those having forty-five or more should have chromosomes which are obviously shorter. A comparison of a cell having many chromosomes with others in the same tissue, or in other tissues having the spermatogonial number or very close to it, will show that the chromatin bodies in the many-chromosome cells are in general, shorter. Compare figures 23 to 80 with figures 1 to 20 and the photomicrographs 96, 97 and 98 with 99. With the ocular evidence in favor of the breaking up of the somatic chromosomes, further evidence indicating that such a process is actually taking place may be offered. That the 'extra' chromosomes...
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    Contributions from the Zoological Laboratory of the University of Pennsylvania Volume 20

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