470 FREDERICK KEEBLE. 
tive bands undergoes marked reduction in amount, and 
frequently almost entirely disappears at the time of egg- 
laying. The stimulatory action of added nitrogen on egg- 
production may be, and probably is, due to the transmission ~ 
from the algal cell to the animal, not only of the fatty 
assimilate, but also of products of proteid synthesis. 
It is possible, however, that this increase in egg-production 
is an indirect rather than a direct consequence of added 
nitrogen. It is known from Boussingault’s classical researches 
(1860) that nitrogen-starvation reduces enormously the photo- 
synthetic activity of the green plant. So here it may be that 
the poor results in “ filtered sea-water, light,” and the good 
results of ‘filtered sea-water and added nitrogen, light”? are’ 
due, the former to the curtailment of fat-synthesis by the 
algal cell in the nitrogen-starved animal; the latter to the 
active fat-synthesis by the algal cell well supplied with nitro- 
gen. However this may be, the two sets of experiments just 
described serve to account for the luxuriant development of 
the yellow-brown cells within the body of C. paradoxa. In 
their free state these algee, like all marine plants, run grave 
and frequent risks of nitrogen-starvation or, at all events, 
have their increase limited by the shortage of available 
nitrogen. Wherever there is any leakage of nitrogen in any 
form—and traces of combined nitrogen must be given off 
from all such animals as the Accelous Turbellaria—it is to be 
presumed that marine, motile plants will congregate. Con- 
gregating about C. paradoxa they will be ingested, and if, 
as happens in the case of the yellow-brown cell, they withstand 
digestion, they find rich stores of nitrogen—the waste nitro- 
gen of the animal’s metabolism—at their disposal. Thus they 
solve the problem of how to obtain sufficient nitrogen. To 
the individual infecting organism it is a solution; to the 
species it is none, for in effecting the solution to this nitrogen- 
problem the yellow-brown cell dooms itself to death with- 
out issue. It is to be noted that the association between 
aloal cell and animal, though it has precisely the same signi- 
ficance in C. roscoffensis as in C. paradoxa, has gone a 
