i8o AGE, GROWTH, AND DEATH 



reproduction would be impossible ; that the cells of 

 the body, having been so changed, as we have seen, 

 are no longer capable of returning backwards along 

 the path they have journeyed ; they can only remain 

 where they are, or go yet farther onward in the career 

 of cytomorphosis. Nature, however, has met this 

 difficulty by a way which we have only recently dis- 

 covered. We are not yet sure that the way we have 

 discovered is the only way, that it is the universal 

 method in the case of all animals for accomplishing 

 the purpose. The discovery of this method of pro- 

 viding for the perpetuation of youthfulness from one 

 generation to another is due to the investigations of 

 Professor Nussbaum, of Bonn. 1 The theory which 

 he put forward has been verified by subsequent 

 examinations and investigation, and confirmed, I am 

 glad to say, in part by some very interesting and 

 careful observations which have been made here in 

 Boston by Dr. F. A. Woods, 2 at that time a member 

 of my laboratory staff. Perhaps the very best con- 

 firmation of all is the recent extension of our know- 

 ledge in regard to this theory which comes from the 

 investigations 3 of Dr. B. M. Allen, made at Madison, 

 on the perpetuation of germ cells in the developing 



1 M. Nussbaum, " Ueber die Veranderungen der Geschlechtsprodukte bis zur 

 Eifurchung ; ein Beitrag zur Lehre der Vererbung," Archiv. fur Mikrosk 

 Anat., xxiii., 155 ; cf. also xli., 119. 



8 F. A. Woods, "Origin and Migration of the Germ-cells in Acanthias^ 

 American Journ. of Anat., vol. i, p. 307. 



3 Bennet M. Allen, Science, vol. xxi., p. 850 (1905); Amer. Journ. of 

 Anatomy, vol. v., pp. 79-94 ; Anatomischer Anzeiger, xxix., 217, and xxx., 391 ; 

 and on the frog, see Anatomischer Anzeiger, xxxi., 339-347 (1907). 



