Page:Popular Science Monthly Volume 71.djvu/464

458 a special interest to naturalists, owing to the fact that in its early development it offers features of resemblance to man which are very striking and instructive. The plate is from a series of drawings made under the direction of Professor Hubrecht, the principal student of the development of this type of animal. Here (No. 1) we can see an early stage in which the germ consists of but a single cell, and at this point is the nucleus. Note its size and then compare it with the nuclei in Nos. 2 and 3 in which several of these cells, as they appear in a section, are represented. The cells themselves are now smaller because they have multiplied by the division of the original germ, but the nuclei in them are likewise smaller. And in the older stage, No. 3, where the number of cells has begun still further to increase, we see that there is another and more marked reduction in the size of the nuclei. Contrast the single nucleus of the early stage with the small nuclei of the later one, and notice how very striking is the change in the size. Thus during the early development of the individual, and it seems to be true of all animals, we find that there is an actual rapid reduction in the size of the nucleus. As we have learned that the proportion of the nucleus and the protoplasm is so important, we must attribute to this alteration in the dimensions of the nucleus great significance.

We have next a series of figures which have interested me very much and which I only recently secured as the result of studies I have been making in my own laboratory at the Harvard Medical School. These pictures are now shown publicly for the first time, and record a fact which, so far as I know, has never yet been clearly noted and recognized as important by any investigator. The four figures at the top represent four single nuclei taken from different parts of a rabbit seven and one half days after the commencement of its development. The second set of figures, 5, 6. 7 and 8. show nuclei from different characteristic parts of a rabbit embryo of ten days. Note, please, the size of these nuclei, the curious network of threads in their interior and the existence, generally more or less in a central position, of a mass of material which stands out conspicuously and represents a condensation of the nuclear stuff at that particular point. Such a central body is highly characteristic of these early stages. Next we come in the series of figures, from to 20, stretching across the screen in two lines, to a rabbit embryo of twelve and one half days. Instead of having nuclei of large size we have now nuclei which are obviously small. Instead of having nuclei which are more or less alike in appearance, we have now nuclei of great diversity. Every one of these figures, as you will readily see if you run your eye along from one end of the lines to the other, has a distinctive character of its own. In this period, then, of two and one half days, there has been a revolution in the character of the nuclei of the developing embryo. Where before the nuclei were alike, now they have become unlike. Two of these I