Report of the Commissioner - United States Commission of Fish and Fisheries Volume 10

Report of the Commissioner - United States Commission of Fish and Fisheries Volume 10

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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. 1884 edition. Excerpt: ...layer of the latter, and in this layer four free nuclei may now be noted. The cleavage or segmentation now proceeds regularly. When the nuclei of the archiblast have been transformed into new spindles, the same takes place with the nuclei of the parablast. All of the free nuclei are over undergoing similar phases of division, or are synchronously in a state of rest, and the nuclei of the segments of the archiblast pass through the same phases, at least during the first few hours of segmentation, whilst at a later period the free nuclei of the parablast pass into a resting stage. "With the completion of segmentation, the Teleostean egg consists of a great number of segmentation spheres, comprising the segmented germdisk or archiblast, and the multinucleated parablast. From the archiblast all of the germinal layers are developed; never does the parablast take part in their formation. That this is the fact is best shown by means of sections, prepared from embryos in much more advanced stages of development, in which the intestine has been developed, and in which we still find these same free nuclei as at first. We may then rightly inquire what is the significance of the protoplasmic covering of the food yelk in which a great multitude of free nuclei are imbedded. The only answer which I can give to this question is the following: The parablast, so rich in free nuclei, assimilates the constituents of the food-yelk, in order to convert them Into a form suitable for the growth of the cells of the archiblast, or to convert the yelk into the embryonic layers developed from the archiblast; in other words, the multinucleated parablast assumes the rdle of provisional blood. This view I would justify by the three following sets of facts: (1.) more

Product details

  • Paperback
  • 189 x 246 x 24mm | 830g
  • United States
  • English
  • black & white illustrations
  • 1236775635
  • 9781236775634