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Updated: May 3, 2025
This would be true no matter how much this undifferentiated material should increase in amount by assimilation, so long as it remained unaltered in character, and it hence follows that every individual carries around a certain amount of undifferentiated chromatin material in all respects identical with that from which he developed.
Thus we see that part of the chromatin material in the egg of the first generation develops into the second generation, while another part of it remains dormant in that second generation, eventually becoming the chromatin of its eggs and spermatozoa.
It thus follows that the adult will contain, along with its differentiated material, a certain amount of the original physical basis of heredity which still retains its original powers. This undifferentiated chromatin material originally possessed powers of producing a new individual, and of course it still possesses these powers, since it has remained dormant without alteration.
This gives the physical basis for paternal inheritance as well as for maternal inheritance, and it shows why they may be of the same or equivalent degree. When, now, the egg divides, at the first and later cleavages, the chromatin masses or chromosomes contained in the double nucleus are split lengthwise and the twin portions separate to go into the nuclei of the daughter-cells.
When the germ-cells of the male and female make the division which marks the first step in reproduction, however, the process is different. Half the chromatin material passes into each of the two cells formed. This is called maturation, or the maturation division, and the new cells have only half the original number of chromosomes.
Now, if this chromatin thread consists of a series of units, each representing certain hereditary characters, then it is plain that the division of the thread by splitting will give rise to a double series of threads, each of which has identical characters.
The cell division is so arranged that each new cell receives an equal share of the male and female chromatin, and this process is continued in every case of cell division, so that eventually, in every part of our bodies, the dual inheritance remains complete.
The chromatin of the nucleus contains the determinants of hereditary qualities. In reproduction, the male sex-cell, which is scarcely more than a minute mass of chromatin provided with a thin coat of protoplasm and a motile organ, fuses with the egg, and the nuclei of the two cells unite to form a double body, which contains equal contributions of chromatin from the two parental organisms.
The cell, therefore, possesses a nucleus which has the power of enabling it to assimilate its food that is, to convert it into its own substance. The nucleus further contains a marvellous material chromatin which in someway exercises a controlling influence in its life and is handed down from one generation to another by continuous descent.
To be sure there is a remarkable substance, called chromatin because of its capacity for taking up certain dyes, which evidently plays some profoundly important part in the processes of development. We may suspect that this is the thing which carries the physical characteristics from one generation to another, but we cannot prove it; and though some authorities think that it is, others deny it.
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