DIFFERENTIATION AND REJUVENATION 163 



nucleus, with the figure on the right having a number 

 of nuclei. Both figures represent the very earliest 

 stages of development and show the full size of the 

 whole germ, which is about the same in the two stages. 

 The total amount of living material has not changed 

 essentially, but evidently there has occurred a marked 

 increase of the nuclear substance. The nuclei have 

 in the right-hand figure multiplied in number and 

 their combined volume is much greater than the total 

 volume of the single nucleus in the left-hand figure. 



The increase in the amount of nuclear material dur- 

 ing the segmentation of the ovum occurs in all classes 

 of animals and has been recorded by hundreds of 

 observers. It has hitherto attracted very little at- 

 tention and despite the constancy and universality of 

 the phenomenon no special significance has been at- 

 tributed to it. I emphasised the constancy of the 

 phenomenon in 1890 in an address delivered before 

 the American Association for the Advancement of 

 Science. 1 Since then Richard Hertwig has been the 

 only author to lay special stress on the fact, but he 

 fails to make the interpretation, which I shall offer 

 you in a few moments. 



We can get a further notion of the nuclear increase 

 by studying the very early development of a salaman- 

 der (Fig. 59). Here upon the screen is the egg of a 

 salamander, No. i. It represents really but a single 

 cell. It then divides into two cells : each of those 

 cells has a nucleus which we cannot see because these 



1 Proc. A, A. A. S., xxxix. 



