Text-book of embryology / by Frederick Randolph Bailey and Adam Marion Miller.

  • Frederick R. Bailey
Date:
1912
Catalogue details

Licence: In copyright

Credit: Text-book of embryology / by Frederick Randolph Bailey and Adam Marion Miller. Source: Wellcome Collection.

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    only for some resting cells, is always present in cells which are undergoing mitotic division. In addition to the net knots are the true nucleoli or plasmosomes. These are spheroidal bodies which lie free in the meshes of the nuclear reticulum. They vary in number in different cells and sometimes in the same cell in different conditions of activity. They stain intensely with basic dyes. The function of the nucleolus is not known. It has been regarded by some as material in process of constructive metabolism, by others as a waste product. The nucleus is typically spherical. Its shape may or may not be modified by the shape of the cell body. Nuclei may assume very irregular shapes, as in polymorphonuclear leucocytes, or they may be lobulated, as in some of the Cell membrane Metaplasm ) granules j Karyosome or ) net knot J Hyaloplasm Spongioplasm Linin network Nucleoplasm Fig. i.—Diagram of a typical cell. Bailey. Aster (attraction-sphere) Centriole Plastids (metaplasm) Chromatin network Nuclear membrane Nucleolus Vacuole large cells of bone marrow; or a cell may have a number of nuclei. The shape of the nucleus may vary considerably within comparatively short periods of time, Such nuclei have been described as having amoeboid movement. The size of the nucleus also appears to be independent of the size of the cell body, some large cells having small nuclei, while some small cells are almost completely filled by their nuclei. The nucleus tends to lie near the center of the cell, yet may be eccentric to any degree and appears to be suspended in the cytoplasm in such a way that its location within the cell may change. In some of the lowest forms no true nuclear structure exists, scattered granules of chromatin consti- tuting the rudimentary nucleus, generally called a diffuse nucleus. As the nucleus is an essential element in all reproduction, it follows that all cells have been nucleated at some time in their developmental history, and that the adult nonnucleated condition of some cells (e.g., respiratory epithelium) is indicative of their having passed beyond the age of reproductive power. If the nucleus be removed from a living cell, the cytoplasm does not necessarily
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