1 50 PLANT- ANIMALS [CH. 



acid proved that the latter were, after twenty-one 

 days, considerably larger than the former. 



The experiment was continued. During the follow- 

 ing weeks the animals in filtered water, dwindled, lost 

 all their yellow-brown cells, became of microscopic 

 size and died. On the other hand, after upwards of 

 thirteen weeks, specimens of the animals in filtered 

 water plus uric acid were alive, of a recognisably 

 brown colour and possessed of many normal, yellow 

 brown cells. 



We thus have proof that when C. paradoxa is 

 kept in the light, so that its yellow-brown cells may 

 photosynthesise, and when uric acid is supplied, this 

 substance serves as a source of nitrogen to the yellow- 

 brown cells. Moreover, in these circumstances, the 

 materials manufactured by the yellow-brown cells 

 serve not only for the nutrition of the alga but also 

 for that of the animal. This, however, means that 

 the yellow-brown cells contribute not only fatty but 

 also nitrogenous, protein-forming material to the 

 animal. That this is the case the results of the 

 second mode of experimentation render highly 

 probable. 



Here, in lieu of determining the effect of uric 

 acid on the life of algal cell and animal, its influence 

 on egg-laying was investigated. The experiment 

 consisted in maintaining equal numbers of similar 

 animals in filtered sea- water, under conditions which 



