482 



Growth of Mammalian Tissue in Vitro. 



that in such cases the plasma is always improved if it he frozen for a period 

 of about three days, which is very suggestive that the poor growth is due to 

 the presence of an inhibiting substance. It is probable, therefore, that in 

 young animals, also, the inhibiting substance is present in larger quantities, 

 and hence the plasma makes a poor medium. 



Conclusions. 



1. Growth of tissues in vitro affords a valuable means of investigation as to 

 the effects of age upon growth. 



2. The tissues of young animals grow more rapidly and vigorously than 

 those of adult animals. 



3. The plasma of young animals is a much less suitable medium for the 

 growth of tissue in vitro than the plasma of old animals. 



4. The unsuitability of the plasma of young animals as a medium is probably 

 due to the presence of an increased amount of some inhibiting substance. 



EXPLANATION OF PLATE. 



Growth of Fresh Liver. 



Fig. 1. — Two days' growth of young liver in old plasma. 

 Fig. 2.— Two days' growth of young liver in young plasma. 

 Fig. 3. — Two days' growth of old liver in old plasma. 

 Fig. 4. — Two days' growth of old liver in young plasma. 



Growth of Stock Testicle. 



Fig. 5. — Two days' growth of testicle in plasma plus spleen extract. 

 Fig. 6. — Two days' growth of testicle in old plasma. 

 Fig. 7. — Two days' growth of testicle in young plasma. 



