REGENERATION AND DEATH 171 



cells from the embryonic cells of the nervous system 

 has been amply proved ; secondly, that at the time the 

 nerve cells arise no outside cells enter the nervous 

 system ; and, thirdly, that the nerve cells are produced 

 in mammals before there are any leucocytes present 

 in the embryo. You need hardly take Kronthal's 

 theory seriously. 



As a preliminary to the discussion of this interest- 

 ing phenomenon, it is necessary to say a few more 

 words in regard to the nuclei. We have learned that 

 the units out of which the body is constructed, the 

 cells, consist each of a little mass of protoplasm with 

 a central body called the nucleus : and we have also 

 learned that the increase of the protoplasm and the 

 subsequent differentiation of the cell is to be looked 

 upon as the cause of old age, and the increase of the 

 nucleus as the cause of youth, of rejuvenation. In 

 addition to what has been said concerning the size of 

 the nucleus, some further explanation is necessary, 

 and that can best be given with the aid of illustrations 

 upon the screen. The first of the pictures, Fig. 59 

 (seep. 164), may serve to recall to your minds what I 

 said in regard to the process of the segmentation of 

 the ovum. Here is an ovum, No. i, a single cell, but 

 relatively of enormous size ; it is the ovum or germ of 

 a newt, and the plate illustrates to us the gradual pro- 

 cess of division of the original single cell into a num- 

 ber of distinct cells, each of which we call a segment, 

 and the formation of them we call segmentation, a 

 name which we keep from the olden time when the 



