The Organization of the Egg 201 



growth and keeps the animals healthy. The food supply 

 must also exercise a controlling influence, the growth of 

 bacteria, which serve as food, being also checked by waste 

 products (toxins) in the culture. It can be shown however 

 that even in the presence of abundant food supply, stale 

 culture will inhibit the growth of Paramoecia. 



We can compare the metazoan with a culture of Paramoecia, 

 all descendants of one cell. The former, as well as the latter, 

 starts as a single cell (the fertilized egg). After a period of 

 active division or growth the climax is reached, when the 

 processes of repair can only keep pace with those of waste, 

 and from then on the organism passes through the decline 

 of old age followed by death. But if a few cells be removed 

 from the parent body and transferred to a fresh medium 

 (blood plasma) they forthwith start to grow abundantly, and 

 this growth can apparently be maintained indefinitely, if the 

 transfers be repeated from time to time. 



The influence of the age of the animal from which the 

 plasma is taken is very marked. In that from young animals 

 growth is much more active than in that taken from adults, 

 but if an extract of the tissues of a young animal be added 

 to the latter the growth is materially increased. 



May not then old age and death be caused by waste prod- 

 ucts excreted by the cells of the metazoan body? May it 

 not be a process of auto-intoxication, not localized as Metsch- 

 nikoff suggests, in the intestines, but generalized throughout 

 the entire body ? Whatever answer to this question the future 

 may make, the faculty of unlimited growth possessed by most, 

 if not all the tissues of higher animals, suggests not only the 

 indeterminate nature of development, but also the inherent 

 immortality of the cell. 



