Daedalus; Proceedings of the American Academy of Arts and Sciences Volume 30
This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1895 edition. Excerpt: ...with a germinal disk 0.28 mm. in diameter, the lateral proliferations seem to have been entirely suppressed, and we have an overgrowth from only one region, the posterior, extending forward along the median line of the disk. The fate of the bridge seems to be, that its free anterior margin finally meets the true ectoderm of the disk; the structure then sinks down until it comes in contact with the underlying ectoderm, with which it finally fuses. At the same time the disk increases in area, this being largely due to a rearrangement of the cells of the disk in consequence of the addition received from the bridge. This method of increasing in size at this stage was first suggested to me by the fact that few nuclei are found in a karyokinetic condition. Accordingly, in the case of the two germinal disks represented by Figs. 5 and 6, Plate I., which correspond to the two stages of development in question, I counted the number of nuclei in each, as seen in sections, to determine the numerical relations of the cells. In the disk of Fig. 5 I found 1067 cells, in that of Fig. 6, 992; although embryos in their early development grow at very different rates, still the facts which these numbers present, together with the absence of nuclear figures, would seem to point to a simple rearrangement of existing cells as the principal factor in the increase in area of the germinal disk at this stage, rather than to an active multiplication of cells. The oldest embryos considered in this paper consist, then, of a blastodermic vesicle, composed of a continuous inner sac of entoderm closely surrounded by a layer of ectodermal cells, which in the germinal disk are thickened into a flat, ovate expanse, without primitive groove or streak, with no signs of any...
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- 13 Sep 2013
- United States
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