Page:Popular Science Monthly Volume 71.djvu/467

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461
AGE, GROWTH AND DEATH

primitive condition, with the protoplasm still undifferentiated and the nucleus of the embryonic or simple organization, and hold them apart from the rest of the body, not separating them so that they come off and leave the body, but so that they have a different history; so that they escape the change which the other cells of the body must pass through. These cells of a simpler character are gathered together, kept asunder, and not allowed to progress in the development of all the cells which form the body proper. We have learned, for instance, that in the development of the dog-fish very early, before any organs exist, cells are formed into a cluster. They lie by themselves, are easily recognized under the microscope, and they have obviously the primitive character which I have endeavored to explain to you. And they remain such. Meanwhile as development progresses, all the remaining cells—all those not part of these clusters, pursue their proper careers, become differentiated; but the cells in the clusters do not change for a long period. Later as the organs become differentiated, we can recognize in the direct descendants of these cells, which have been traced from stage to stage so that their history is known with certainty, those cells which in the adult we call the germ cells, and which are to serve for the reproduction of the species. These cells are set apart at all periods. They represent germinal matter which is withheld from the metamorphosis which the rest of the body undergoes. They have a continuous history. Hence we bestow upon this method, under the conception that it is applied to secure propagation of the species, the term—theory of germinal continuity. It is the theory of hereditary transmission which I think is now universally held by all competent biologists. Our study of nuclei and of their relations to protoplasm serves to clear up in our minds, it seems to me, to some degree at least, the necessity which really exists for this device of germinal continuity, of the setting apart of certain cells of the rejuvenating sort, of the young sort, of the embryonic type (the term you apply to them matters little), which cells are those used to produce the new offspring of the next generation. All this, of course, fits perfectly with the doctrine which I have been telling you of again and again in this course of lectures, that the progress of differentiation is always in one direction and ends in the production of structure which, if it is pursued to its legitimate terminus, results in the degeneration and death of the cell. Obviously such a set of changes as I have thus indicated can not produce the sort of a cell which is necessary for reproduction.

I wish there were time to enter more fully into this question of the size of nuclei, for there is much which might be said concerning it. This much more, however, ought to be said to you—that the problem of the size of nuclei is by no means a simple one. It has been found, for instance, in the experiments made upon some of the simple algæ, the so-called Spirogyra, which every elementary student of botany probably