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Lastly, the cell has the centrosome, which brings about cell division in such a manner that this chromatin material is divided equally among the subsequent descendants, and thus insures that the daughter cells shall all be equivalent to each other and to the mother cell. We must therefore look upon the organic cell as a little engine with admirably adapted parts.
This is the probable explanation of the fact that an athlete or a race-horse trained to the point of highest efficiency can reach his maximum record but once in his life. Under certain conditions, however, it is possible that, though some chromatin is forever lost, the remainder may be so remarkably developed that for a time at least it will compensate for that which is gone. Hemorrhage.
The pursuit of things into the most minute details leads to no indication of it. The chromatin, in which the most important vital processes have their basis, is very far from having this machine-like structure; it is homogeneous.
As of course only the germ-cells of an adult organism pass on to form later generations, and as their content of chromatin is derived not from the sister organs of the body, but from the original fertilized egg, there is a direct stream of the germ plasm which flows continuously from the germ-cell to germ-cell through succeeding generations.
As the same process seems to hold for all the later divisions of the cleavage-cells whose products are destined to be the various tissue elements of the adult body, it follows that all tissue-cells would contain chromatin determinants derived equally from the male and female parents.
Suppose we could go further than is now possible and could lay bare the ultimate make-up of the chromatin of these one-celled creatures, would we even then be able to prove that life with all its properties is inherent in these material components of the cells? In other words, would we really solve anything after all?
It remained for Weismann to identify this theoretical substance with a specific part of the cell, namely, the deeply staining substance, or chromatin, contained in the nucleus of every cell.
This process of the formation of the polar cells is thus simply a device for getting rid of some of the chromatin material in the egg cell, so that it may unite with a second cell without doubling the normal number of chromosomes. Completion of the process of extrusion of the chromatic material; fn shows the two chromosomes retained in the egg forming the female pronucleus.
If a given character is represented by a portion of the chromatin in the original ovum, this has to be divided so many times, and each time to grow to the same condition as before. How can we suppose that the divisions shall be exactly equal or the growth always the same?
Of all parts of the cell this chromatin is the most remarkable. It appears in great variety in different cells, but it always has remarkable physiological properties, as will be noticed presently. All things considered, this chromatin is probably the most remarkable body connected with organic life.
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