Page:Popular Science Monthly Volume 71.djvu/371

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365
AGE, GROWTH AND DEATH

alone, but the protoplasm in the interior of the cell has become in part changed into other things. Here then within the territory of a single cell we have differentiation. If now in these unicellular organisms we study both the protoplasm and the nucleus, we learn that most of these modifications which are so conspicuous upon microscopic observation are due to changes in the protoplasm. It is the protoplasm which acquires a new structure. In the nucleus, on the contrary, we find perhaps a change of form, minor details of arrangement by which one sort of nucleus, or one stage of the nucleus, can be distinguished from another, but always the nucleus consists of the same fundamental constants. There is the membrane bounding it; there is the sap or juice in the interior; the network of living threads stretching across it: and here and there imbedded in and connected with this network are the granules of special substance, which we call chromatin. These four things exist in the nuclei and are apparently always present, and there is usually not to be seen in the nucleus anything of change comparable, in extent at least, with the change which goes on in the protoplasm—on the other hand, the protoplasm acquires items of structure which were totally absent from it before. The nucleus rearranges its parts rather than changes them. This is a very important fact, and shows us, if we confine our attention even to these little organisms only, that the differentiation of the protoplasm is quantitatively the more important of the two—the differentiation of the nucleus the less important.

We can now turn from a consideration of these lowest organisms to the higher forms, among which we ourselves of course are counted, in which the body is formed by a very considerable number of cells. Again I should like to take advantage of your kindness and show you some of the pictures we have already reviewed, in order to utilize the features which they show as illustrations of the fundamental principle that the conspicuous change is in the protoplasm. Here we have nerve cells. In the first two photographs are represented two isolated nerve cells, to show their shape. They have been colored by a special process so dark that the nucleus which they contain in their interior is hidden from our view; it is of course none the less there. This dark staining enables us to trace out the shape of these cells very clearly, and you can see that instead of being round and simple in form they have their elongated processes stretching out to a very considerable distance; these processes serve to catch up from remote places nervous impulses and carry them into the body of the cell, and thus assist in the work of nervous transmission. The elongation of these threads is, as you see. adapted, like the elongation of a wire, to long-distance communication. Here are two other figures which represent nerve cells treated by a different process, and again artificially colored. But the color in this case has attacked certain spots in the protoplasm.