Anatomy, descriptive and applied / Henry Gray. A new American from the 18th English ed., thoroughly rev. and re-edited, with the Basle anatomical nomenclature in English, by Robert Howden.

  • Gray, Henry, 1827-1861.
Date:
1913
Catalogue details

Licence: Public Domain Mark

Credit: Anatomy, descriptive and applied / Henry Gray. A new American from the 18th English ed., thoroughly rev. and re-edited, with the Basle anatomical nomenclature in English, by Robert Howden. Source: Wellcome Collection.

Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.

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    meeting near the centre of the yolk, fuse to form a new nucleus, the segmentation nucleus, which therefore contains both male and female nuclear substance; the former transmits the individualities of the male ancestors, the latter those of the female ancestors, to the future embryo. By the union of the male and female pronuclei the number of chromosomes, is restored to that which is present in the nuclei of the somatic cells. SEGMENTATION OF THE FERTILIZED OVUM. The early segmentation of the human ovum has not yet been observed, but judging from what is known to occur in other mammals it may be regarded as certain that the process starts immediately after the ovum has been fertilized, i. e., while the ovum is in the uterine tube. The segmentation nucleus exhibits the usual mitotic changes, and these are succeeded by a division of the ovum into two cells of nearly equal size.^ The process is repeated again and again, so that the two cells are succeeded by four, eight, sixteen, thirty-two, and so on, with the result that a mass of cells is found within the zona striata, and to this mass the term Fig. 67.—First stages of segmentation of a mammalian ovum. Semidiagrammatic. (From a drawing by Allen Thomson.) z.p. Zona striata, p.gl. Polar bodies, a. Two-cell stage. 6. Four-cell stage, c. Eight-ceU stage. d, e. Morula stage. morula is applied (Fig. 67). The segmentation of the mammalian ovum may not take place in the regular sequence of two, four, eight, etc., since one of the two first formed cells may subdivide more rapidly than the other, giving rise to a three- or a five-cell stage. The cells of the morula are at first closely aggregated, but soon they become arranged into an outer or peripheral layer, the trophoblast, which does not contribute to the formation of the embryo proper, and an inner cell-mass, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into 1 In the mammalian ova the nutritive yolk or deutoplasm is small in amount and uniformly distributed through- out the cytoplasm; such ova undergo complete division during the process of segmentation, and are therefore termed holoblastic. In the ova of birds, reptiles, and fishes where the nutritive yolk forms by far the larger portion of the egg, the cleavage is limited to the formative yolk, and is therefore only partial; such ova are termed meroblastic._ Again, it has been observed, in some of the lower animals, that the pronuclei do not fuse but merely he in apposition. At the commencement of the segmentation process the chromosomes of the two pronuclei group themselves around the equator of the nuclear spindle and then divide; an equal number of male and female chromosomes travel to the opposite poles of the spindle, and thus the male and female pr9nuclei subscribe equal shares of chromatin to the nuclei of the two cells which result from the subdivision of the fertihzed ovum.