< Page:Popular Science Monthly Volume 83.djvu
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190
THE POPULAR SCIENCE MONTHLY
Replicated Ref# 14 (Conklin, "Cell Size and Nuclear Size," Jour. Exp. Zool., 12, 1912.) (Wikisource contributor note)

muscle fiber. These facts seem to me to justify the conclusion which I reached in a former paper:[1]

It is probable that the contractile substance which makes up the larger part of the muscle cell does not contribute to the growth of the nucleus as does the protoplasm of embryonic cells—that so far as the growth of the nucleus is concerned it acts as does yolk, oil, membranes, fibers or other products of metabolism and differentiation. If only the sarcoplasm of the muscle cell and not its contractile substance is able to contribute to the growth of the nucleus, the small volume of the nuclei as compared with the entire cell would find a ready explanation. There can be no doubt that the plasma is the chief seat of differentiation, as Minot has emphasized, and that highly differentiated cells, such as muscle, nerve, and some kinds of connective tissue, have a larger amount of plasma and its products, relative to the nucleus, than have embryonic cells. In the case of fiber cells, fat cells and probably muscle cells, the cell body becomes filled with the products of differentiation and metabolism, which like the yolk in egg cells, or the secretion products in liver cells can not enter the nucleus and consequently do not influence its size. In such tissue cells the cell body is relatively much greater as compared with the nucleus, than in purely protoplasmic cells, but I have been unable to find any evidence that the ratio of protoplasm (using this term in its usual sense) to the nucleus is greater in tissue cells of Crepidula than in the blastomeres.

Just as the size of a nucleus in any given species is proportional to the volume of the general protoplasm, so the volume of its chromosomes is proportional to the volume of the nucleus. The number of chromosomes and their relative sizes are characteristic for each species, but the absolute size of chromosomes depends upon the size of the nucleus from which they come. Throughout the period of cleavage, as the cells and muclei grow smaller, the chromosomes also diminish in size. The view of Boveri[2] that the chromosomes divide when they have grown to their original size before division, and that thereby a definite specific size of ihe chromosomes is maintained, finds no confirmation in the work of Erdmann,[3][4] Schleip[5] or myself; while the view of Koehler[6] that the autonomy of the chromosomes may be extended to their growth, which is supposed to be independent of that of other cell constituents, is flatly contradicted by the facts.

During the cleavage stages at least, neither the nuclei as a whole nor the chromosomes double in volume at each successive division as is so often assumed. The total volume of the nuclei at the 70-cell stage of Crepidula plana is only 2.25 times their volume at the 2-cell stage. The

  1. ↑ Conklin, "Cell Size and Nuclear Size," Jour. Exp. Zool., 12, 1912.
  2. ↑ Boveri, Zellenstudien V., Jena, 1905.
  3. ↑ Erdmann, "Experimentelle Untersuchungen der Massenverhältnisse, etc.," Arch. Zellforsch., 2, 1908.
  4. ↑ Erdmann, "Qualitative Analyse der Zellbestandteile, etc.," Ergeb. Anat. Entw., 20, 1912.
  5. ↑ Schleip, "Das Verhalten des Chromatins, etc.," Arch. Zellforsch., 7, 1911.
  6. ↑ Koehler, "Ueber die Abhängigkeit der Kernplasmarelation, etc.," Arch. Zellforsch., 8, 1912.
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