An introduction to the study of embryology / by Alfred C. Haddon.

  • Alfred Cort Haddon
Date:
1887
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to the study of embryology / by Alfred C. Haddon. Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

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    The resulting nuclear spindle is placed vertically, the peripheral nuclear star, or aster, being situated in a small protuberance from the surface of the ovum. This process is segmented off from the ovum, and a minute cell is formed, containing a portion of both the protoplasm and the nucleus of the parent-cell (fig. 7, F and l). Fio. 7.—Formation of Polar Cells in a Star-fish (Asterias glacialis). [Froin Geddes, A-K after Fol, L after 0. Hertwig.\ A. Ripe ovum witli excentric germinal vesicle and spot; B-D. Gradual meta- morphosis of germinal vesicle and spot, as seen in the living egg, into two asters; F. Formation of first polar cells and withdrawal of remaining part of nuclear spindle within the ovum; G. Surface view of living ovum in the first polar cell; H. Completion of second polar cell; I. A later stage, showing the remaining internal half of the spindle in the form of two clear vesicles ; K. Ovum with two polar cells and radial stiise round female pronucleus, as seen in the living egg. [E, F, H, aud I, from picric acid preparations.] L. Expulsion of first polar cell. This phenomenon is repeated, and two cells are budded off from the ovum; these are known as the ;polar cells (or as polar bodies, polar globules, directive bodies, &c.), from the fact that they are invariably derived from that pole of the ovum at which the epiblast or upper-layer cells will be developed; hence, also, this pole is Fig. 8.—Formation of Polar Cells IN Ovum of Elvsia viridis. Tlie upper pole of the ovum becomes amoeboid during the formation of the polar cells. The second polar cell is in process of formation. usually termed the upper pole of the ovum (see figs. 12 and 17). During the production of the polar cells, the ovum, especially at its upper pole, may exhibit amoeboid movements; this is well shown in^the ovum of Elysia (fig. 8). Although the polar cells may remain attached to the developing
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