REGENERATION AND DEATH 177 



have now nuclei of great diversity. Every one of 

 these nuclei, 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 devel- 

 oping embryo. Where before the nuclei were alike, 

 now they have become unlike. Two of these I should 

 like especially to call your attention to, because they 

 are the nuclei of the nerve cells this one, No. n, 

 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 drawings 

 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, 27, 28, 

 nuclei of the excretory organ, a nucleus of the con- 

 nective tissue, 24 ; we have nuclei from the lining of 

 the wind-pipe, 32; and the lining of the gullet, 31. 

 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 ac- 

 quired a great diversity in the structure of the nuclei 

 as well as in that of the protoplasm, of which we have 



