External Factors and Size 155 



concentrations from pH 6.4 to 8.0 there were always 

 losses of volume, for the individuals came into the 

 solutions from nearly pure water. The losses were 

 most rapid at pH 7.0. But in pH 6.4 to 6.0 the body 

 volumes always increased. The modifications of vol- 

 ume after complete acclimatization to the various acidi- 

 ties, as in succeeding generations, were not recorded; 

 they might be very different from the initial changes 

 which were observed. 



Attention is eventually also required to the corres- 

 pondence and lack of correspondence between the 

 hydrogen ion concentration of the body substances 

 and that of the medium, and its effects. 



Metabolic products. Apparently nothing is known 

 with respect to the influence of specific chemical com- 

 pounds, either inorganic or organic, upon body sizes. 

 Yet it is generally supposed that the accumulation of 

 some metabolic products, possibly organic, is the ef- 

 fective factor of ageing in a culture medium. The 

 different types of substances which are end-products 

 of metabolism in organisms are not so numerous that 

 they cannot be tested with respect to their effects, both 

 separately and in combination, both upon fission rate 

 and upon body size. 



Trypanosome hosts. The apparent complexity of 

 the sort of analysis of various conditions which is ulti- 

 mately required, may be pictured from the work with 

 parasitic trypanosomes. Wendelstadt and Fellmer 

 ('10) infected various species of cold-blooded animals 

 with Trypanosoma brucei from rats. In the blood of 

 grass-snakes the infecting individuals were much 

 smaller after four days of infection, especially at the 

 eighth day, than they were in the blood of the rats. 

 When put back into rats from the snakes, the parasites 

 were much larger than in the original rat infection. 

 Moreover, the large size persisted not only for many 



