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Updated: May 3, 2025
Weismann, who has most fully elaborated a hypothetical scheme of the relations of the intracellular units, identifies the larger of these units not with the ordinary granules of the cell, but with a remarkable structure called chromatin, which becomes aggregated within the cell nucleus at the time of cellular division a structure which divides into definite parts and goes through some most suggestive manoeuvres in the process of cell multiplication.
Geddes and Thomson and others have maintained that the sex-difference is one of metabolism, the ovum being more anabolic, the sperm more katabolic. A double quantity of special chromatin may be the cause of the greater anabolism of the ovum.
The first indication of the cell division is shown by the chromatin fibres. But whatever be its form during the resting stage, it assumes the form of a thread as the cell prepares for division. It is an extremely important fact that the number of these chromosomes in the ordinary cells of any animal or plant is always the same.
It is thought, however, that a part of the original chromatin material does not thus become differentiated, but remains entirely unchanged as the individual is developing. This chromatin material may increase in amount by assimilation, but it remains unchanged during the entire growth of the individual.
A part, greater or less, of each may be left mixed with the other. This is the probable explanation of the fact that the recessive white plumage has some of the pigment from the dominant form. Segregation, the repulsion between chromosomes, or chromatin, from gametes of different races may occur in different degrees from complete segregation to complete mixture.
The essential object of this complicated phenomena of karyokinesis is to divide the chromatin into equivalent halves, so that the cells resulting from the cell division shall contain an exactly equivalent chromatin content. For this purpose the chromatic elements collect into threads and split lengthwise. The centrosome, with its fibres, brings about the separation of these two halves.
Wilson, T. H. Morgan, and Richard Hartwig have therefore suggested that the sex-difference as regards gametes is not a qualitative but a quantitative one. In certain cases there is no evident quantitative difference of chromatin as a whole, but there may in all cases be a difference in the quantity of special sex-chromatin contained in the X element.
Those cells which are to carry on digestive functions when they are formed receive chromatin material which especially controls them in the performance of this digestive function, while those which are to produce sensory organs receive a different portion of the chromatin material.
Plainly, we must conclude that the chromatin material is something of extraordinary importance to the cell, and the centrosome is a bit of machinery for controlling its division and thus regulating cell division.
Each original cell contains a substance which, on account of its being easily colorable with artificial stains, is called chromatin, and this chromatin is believed to be the bearer of the hereditary qualities.
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