Page:Popular Science Monthly Volume 71.djvu/465

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459
AGE, GROWTH AND DEATH


PSM V71 D465 Nuclei from rabbit embryos.png

Fig. 54. Nuclei from Rabbit Embryos. 1-4, age, seven and one half days. 5-8, age, ten days 9-20, age, twelve and one half days. 21-33, age, sixteen and one half days.

should like especially to call your attention to, because they are the nuclei of the nerve cells—this one, No. 11, from the spinal cord and the right-hand one, No. 10, from the cluster of nerve cells upon the root of a spinal nerve. Finally, we have the series of figures from a rabbit of sixteen and one half days represented in the two lower rows, 21 to 33. In these, if you will leave aside from consideration for the moment 22 and 23, which are obviously of a different size, all are now smaller than they were at twelve and one half days. Every one of the nuclei here represented is characteristic. We have here, for instance, nuclei of the excretory organ; a nucleus of the connective tissue; we have nuclei from the lining of the wind-pipe and the lining of the gullet. Every one of them differs from every one of the others pictured. But if we had drawings of a number of nuclei from the same part of the body and same kind of tissue, we should see that they would be essentially similar. We learn then that there is acquired a great diversity in the structure of the nuclei as well as in that of the protoplasm, of which we have seen so many examples in the previous lectures. You will recall, that as regards the size of cells the nerve cells present a noteworthy excep-