474 FREDERICK KEEBLE. 
are capable of re-infection. With re-infection the growth of 
the animal is resumed. 
12. The yellow-brown cells utilize in their constructive 
metabolism the waste products of the nitrogen-metabolism 
of the animal. The waste nitrogen of the animal is not 
excreted but is stored in the body, probably in the form of 
urates. 
15. Animals deprived of solid food, but kept in the light in 
filtered sea-water to which uric acid is added, conserve their 
yellow-brown cells and maintain their lives longer than do 
animals not supplied with uric acid. 
14. Similarly animals provided with uric acid lay many 
more eggs than are laid by animals kept under precisely 
similar conditions, but not supplied with uric acid. 
15. The interpretation of the relation between yellow-brown 
cell and animal depends on the point of view: 
From that of the animal, it is a case of obligate parasitism. 
From that of the species ‘infecting organism,” it is 
an insignificant episode, involving the loss of that, 
probably small, proportion of its members which are 
ingested. 
From that of the individual ingested yellow-brown cell it 
is a solution of the nitrogen problem—a successful method 
of obtaining large supplies of nitrogen. 
