Page:Popular Science Monthly Volume 71.djvu/468

462 has looked at in the laboratory. that by certain artificial conditions, as made in the experiments of Professor Gerassimow, the size of the nucleus can be changed in the cells, and when the size of the nucleus is changed, the size of the cell alters also. And again, we know that the nucleus provides certain chemical supplies for the life and functioning of the cells. This is very strikingly the case, for instance, in regard to the cells which secrete. These, when they give off the material which they have accumulated in their protoplasm as a preparation for the act of secretion, are found not only to reduce the bulk of their protoplasmic bodies, but the bulk of the nuclei as well. And we know again that the size of nuclei may be changed by somatic conditions, by food supply, so that in every generalization reached by the study of the size of nuclei, we must be very circumspect, and not fancy too easily that we have reached a safe conclusion unless we have taken into consideration all the possible factors by which the size may have been varied.

In what I have said to you hitherto in regard to the power of growth, I have directed your attention chiefly to the power of growth as it exists in a cell in consequence of that cell's condition. When the cell is in the young state, it can grow rapidly; it can multiply freely; when it is in the old state it loses those capacities, and its growth and multiplication are correspondingly impeded, and if the organization is carried to an extreme, the growth and the multiplication of the cell cease altogether.

We find, however, that there is something a little more complicated yet to be considered, for it is not merely a question of the capacity of the cells, but also of the exercise of that capacity, which we must deal with. Here comes in a factor which we learn from the study of regeneration. The phenomena of regeneration are very important and very instructive. We shall come to those in a moment. It will make our study of regeneration clearer, more significant, I think, if we pause for a moment to consider certain fluctuations in the natural development of the organism. We see, for instance, in the brain that early the cells begin to assume the character of nerve cells and that thereafter their multiplication ceases. But, curiously, there will be a spot in the spinal cord, for example, where the change of the cells into nerve cells has not taken place, and from that growth will go on. Cells will migrate from that spot and reach their ultimate destination. When the child is born it is very incapable of movement. There is scarcely more than the power of twitching about in a disorderly fashion. Its muscles can contract, to be sure, but any sort of motion that implies a harmonious working together of various muscles, the baby at birth is quite incapable of. This phenomenon is doubtless due to the fact that the cerebellum, the small brain, is as yet imperfectly developed. If we examine the brain of the child at birth. we find at the edge of the cerebellum a line