1896.] On Heredity and Rejuvenation. 95 



From our present standpoint it is a matter of indifference 

 whether the independent food supply comes from the yolk or 

 from the uterus, however important the difference maybe 

 from other points of view. 



It is to be further noted that our interpretation of the sig- 

 nificance of the embryo is also opposed to Weismann's theory 

 of germ plasm, because it emphasizes the importance of the 

 condition as opposed to the assumption of a germinal substance 

 or plasm. This road also leads to the conclusion reached 

 above by other ways, the conclusion, namely: Reproduction 

 involves rejuvenation, and rejuvenation is characterized by 

 the production of cells with little, and that little not differen- 

 tiated, protoplasm. Since rejuvenated cells arise by asexual 

 as well as by sexual reproduction, since they appear in much 

 greater numbers in embryos than in larvae, and since they 

 may be interpolated, as in the pupae of butterflies, in the 

 midst of the development of an individual, we must admit 

 that the hereditary impulse (vcrerbende Kraft) is distributed in 

 very different cells and is probably distributed equally through 

 all cells. Hertwig has reached the same conclusion, with 

 which Weismann's theory of germ plasm cannot be made to 

 agree. 



As Weismann has neglected the problem of rejuvenation, 

 he has necessarily often gone astray in his discussion of phe- 

 nomena in which rejuvenation plays the principal role. One 

 is astonished at the slight attention bestowed on rejuvenation, 

 when one recalls that it is the central problem of all questions 

 of heredity treated by him. 



Rejuvenation is one of the principal phenomena of life, and 

 the rejuvenated condition of the cell is probably an unavoida- 

 ble preliminary of heredity. We know that at least one ana- 

 tomical sign of the rejuvenated condition is to be found in the 

 preponderance of the nucleus in proportion to the protoplasm : 

 a second anatomical sign is found in the structure of the pro- 

 toplasm, which, in young cells always remains without differ- 

 erentiation. The chief physiological sign of rejuvenation in 

 cells which we as yet know is the power of rapid multiplica- 

 tion. Thus, we see, in case of sexual rejuvenation, that the 



