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Updated: May 17, 2025
The halves of the chromosomes have separated to form two nucleii, each with male and female chromosomes. There now follows a division of the nucleus exactly similar to that which occurs in the normal cell division already described in Figs. 28-34.
A German naturalist recognized among lower animals one group whose distinctive characteristic was that they were made of cells without nucleii, giving the name Monera to the group.
The nucleii of different animals and plants all show essentially the characteristics just described.
In other words, we have here a simple understanding of at least some of the features of heredity. This explanation is that some of the chromatin material or germ plasm is handed down from one generation to another, and is stored temporarily in the nucleii of the reproductive cells.
The nucleus is always a direct descendant from the nucleus of pre-existing cells, so that there is an absolute continuity of descent between the nucleii of the cells of an individual and those of its antecedents back for numberless generations.
The typical form appears to be spherical or nearly so; but from this typical form they may vary, becoming irregular or elongated. The form and shape of the chromatin thread differs widely. The nucleii differ also in the number of nucleoli they contain as well as in other less important particulars.
The centrosome may apparently in some cases disappear, but more commonly remains beside the daughter nucleii, or it may move into the nucleus. Thus the final result shows two cells each with a nucleus and two centrosomes, and this is exactly the same sort of structure with which the process began. Viewed as a whole, we may make the following general summary of this process.
Since each of the four chromosomes thus splits, it follows that each of the two daughter nucleii will, of course, contain four chromosomes; two of which have been derived from the male and two from the female parent.
The nucleus controls, though it may not entirely carry on, the constructive metabolism. It is equally clear that the cell substance is the seat of most of the destructive processes which constitute vital action. The cell substance is irritable, and is endowed with the power of contractility. Cell fragments without nucleii are sensitive enough, and can move around as readily as normal cells.
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