Normal histology / by Edward K. Dunham.

  • Dunham, Edward K. (Edward Kellogg), 1860-1922
Date:
1900
Catalogue details

Licence: In copyright

Credit: Normal histology / by Edward K. Dunham. Source: Wellcome Collection.

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    Owing to their affinity for certain coloring matters, the substances composing the nuclear filaments are called chromatin, or chromo- plasm. Tlie hyaline substances making up the rest of the nucleus do not receive those coloring matters, and for this reason and in this situation are called achromatin. These terms are onlv used in •/ a morphological sense and do not specify any definite chemical com- pounds. The behavior of the nucleoli toward dyes is somewhat ditferent from that of the chromoplasm, which leads to the inference that they are of a different chemical nature. Except during cell-division, the nucleus usually lies quiescent within the cytoplasm, but some observers have seen it execute ap- parently spontaneous movements, and it is evidently possible for its position in the cell to vary from time to time. In marked contrast to this apparently dormant state, as far as visible alterations of structure are concerned, is the role j)layed by the nucleus during the reproduction of the cell. There are two modes of cell-division, the indirect and the direct,^’ but they are by no means equivalent to each other. Tlie former, also termed karyokinesis because of the active changes in the nucleus, appears to be the only truly reproductive process. Direct cell-division results in the formation of new cells, but thev seem to lack that perfection of organization which would be required for the complete and indefinite transmission of all the characters of the parent cells. Before entering into a description of karyokinesis, a few words must be said concerning the centrosome. This is an extremely min- ute granule which is usually situated in the cytoplasm not far from the nucleus. It is often surrounded by a thin zone of hyaloplasm which facilitates its recognition among the fibres and nodal points of union of the spongioplasm. The fibres of the latter are also fre- quently arranged in a radial manner for a short distance around the centrosome. But in many instances it is extremely difficult to dis- tinguish the centrosome, and its constant presence in cells is largely a matter of inference. Sometimes the centrosome is double, the two granules Ivins: close to each other and often beino; surrounded by a common clear zone of hyaloplasm. The first step in the process of cell-division by the indirect method, or karyokinesis, is a division of the centrosome into halves (Fig. 15), which separate and pass to opposite points in the cytoplasm. These points are called the poles of the cell, and when the new cen-