Vietnam or Thailand ? Vote for the TOP Country of the Week !
Updated: May 10, 2025
Now when lata is produced it is believed that in the heterotypic division one pair passes into one daughter-cell instead of one chromosome of the pair into each daughter-cell, the other pairs segregating in the usual way. We thus have one daughter-cell with 8 chromosomes and the other with 6. This 6+8 distribution has actually been observed in the pollen mother-cell in rubrinervis.
The only part of the cell in which complete continuity from parent to child is demonstrated, is the nucleus and particularly the chromosomes. All of these facts simply emphasize the importance of the chromosomes, and tell us that these bodies must be regarded as containing the most important features of the cell which constitute its individuality.
But whatever their number, the chromosomes are always exactly bisected before the cell divides, one-half being apportioned to each of the two cells resulting from the division.
One of each pair goes into one daughter-cell and the other into the other, but not all maternal into one and all paternal into the other. Thus each daughter-cell after the first or heterotypic division in normal cases contains 7 chromosomes. A second homotypic division takes place in which each chromosome splits into two as in somatic divisions, and thus we have 4 gametes with 7 chromosomes each.
According to Mendelism the structure and development of each part of the soma is due to the constitution of the chromosomes of the nuclei in that part. The effects of castration show that the development of certain characters is greatly influenced in some way by the presence of the testes in a distant part of the body.
As a result the next generation would have a larger number of chromosomes in each cell than the last generation, since the division of the egg in development is like that already described and always results in producing new cells with the same number of chromosomes as the starting cell.
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.
All the evidence indicates that these chromosomes carry the "factors" in inheritance which produces the characters or characteristics of the individual body. In mitosis or ordinary cell division, these chromosomes split lengthwise, so that the new cells always have the same number as the original one.
It is a matter of some little interest to have this settled, for if it should prove true, then it would evidently follow that the machinery for cell division, in the case of sexual reproduction, is derived from the father, although the bulk of the cell comes from the mother, while the chromosomes come from both parents.
Word Of The Day
Others Looking