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Old cells which have lost their activity may not show nucleii, but, so far as we know, all active cells possess these structures, and apparently no cell can carry on its activity without them. Some cells have several nucleii, and others have the nuclear matter scattered through the whole cell instead of being aggregated into a mass; but nuclear matter the cell must have to carry on its life.
As the method of studying cells improved microscopists learned better methods of discerning the presence of the nucleus, and as it was done little by little they began to find the presence of nucleii in cells in which they had hitherto not been seen.
In this way the centrosome approaches the female pronucleus, and thus finally the two nucleii are brought into close proximity. In the subsequent figures the chromosomes of the male nucleus are lightly shaded, while those of the female are black in order to distinguish them.
This sort of splitting thus doubles the number of chromosomes, but produces no differentiation of material. Final stage with two nucleii in which the chromosomes have again assumed the form of a network. The next step in the cell division consists in the separation of the two halves of the chromosomes.
A cell cut into three pieces, each containing a bit of the nucleus. Later the experiment was made of depriving cells of their nucleii, and it still further emphasized the importance of the nucleus.
As these two nucleii finally come together their membranes disappear, and the chromatic material comes to lie freely in the egg, the male and female chromosomes, side by side, but distinct forming the segmentation nucleus. The egg plainly now contains once more the number of chromosomes normal for the cells of the animal, but half of them have been derived from each parent.
As microscopists now studied one after another of these animals and plants whose cells had been said to contain no nucleus, they began to find nucleii in them, until the conclusion was finally reached that a nucleus is a fundamental part of all active cells.
As a result of this excessive belief in the efficiency of protoplasm the question of the presence of a nucleus in the cell was for a while looked upon as one of comparatively little importance. Many cells were found to have nucleii while others did not show their presence, and microscopists therefore believed that the presence of a nucleus was not necessary to constitute a cell.
The union of the two pronucleii is plainly to produce a nucleus which shall contain chromosomes, and hence hereditary traits from each parent and the subsequent splitting of these chromosomes and the separation of the two halves into daughter nucleii insures that all the nucleii, and hence all cells of the adult, shall possess hereditary traits derived from both parents.
Huxley, who had first formulated the mechanical theory of life, now startled the biological world with the statement that these collections had shown him that at the bottom of the deep sea, in certain parts of the world, there exists a diffused mass of living undifferentiated protoplasm. So simple and undifferentiated was it that it was not divided into cells and contained no nucleii.
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