Hand-book of physiology / by W. Morrant Baker and Vincent Dormer Harris.

Date:
1888
Catalogue details

Licence: Public Domain Mark

Credit: Hand-book of physiology / by W. Morrant Baker and Vincent Dormer Harris. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    34/910 (page 10)
    been by some described as a separate method, under the title “endogenous fission,” but there seems to be no sufficient reason for drawing such a distinction. It is important to observe that fission is often accomplished with great rapidity, the whole process occupying but a few minutes, hence the comparative rarity with which cells are seen in the act of dividing. Indirect cell division.—In certain and numerous cases the division of cells does not take place by the simple constriction of their nuclei and surround- ing protoplasm into two parts as above described (direct division), but is preceded by complicated changes in their nuclei (karyokinesis). These yj<r 7 —Karyokinesis. a, ordinary nucleus of a columnar epithelial cell; b, c, the same 3 nucleus in the stage of convolution ; d, the wreath or rosette form ; e, the aster or single star; f, a nuclear spindle from the Descemet’s endothelium of the frog’s cornea; o, H,’ i, diaster ; k, two daughter nuclei. (Klein.) changes consist in a gradual re-arrangement of the intranuclear network of each nucleus (see p. 19), until two nuclei are formed similar in all respects to the original one. The nucleus in a resting condition, i.e., before any changes preceding division occur, consists of a very close mesh work of fibiils, which stain deeply in carmine, embedded in protoplasm, which does not possess this property, the whole nucleus being contained in an envelope. The first change consists of a slight enlargement, the disappearance of the envelope, and the increased definition and thickness of the nuclear fibrils, which are also more separated than they were and stain bettei. This is the stage of c on~ volution (fig. 7, B, C). The next step in the process is the arrangement of the fibrils into some definite figure by an alternate looping in and out around a central space, by which means the rosette or wreath stage (fig. 7, d) is reached. The loops of the rosette next become divided at the periphery, and their central points become more angular, so that the fibrils, divided into portions of about equal length, are, as it were, doubled at an acute angle, and radiate Y-shaped from the centre, forming a star (aster) or wheel