IOO2 THE AMERICAN NATURALIST. [Vor. XXXV. 
water. Probably food is actually an important factor, but its 
abundance is also more or less dependent on the temperature 
of the water, so that the latter must be regarded as the most 
important factor in determining the time of egg-laying. 
That food is not of prime importance is clearly shown by a 
large number of observations in the laboratory. In animals 
which contained only small oócytes when taken, and which are 
kept in clear water without food, the growth of the oócytes 
will continue, and within a week or two eggs may be laid. 
This has been found to be the case even during the winter, 
and it shows very clearly that the important factor is the 
degree of temperature which permits the metabolism necessary 
for the formation of the yolk and the growth of the odcyte 
in general. In such cases the oócytes complete their growth 
either at the expense of the other tissues or some store of 
nutritive substance within the body. The body of the animal 
may even decrease somewhat in size during the growth of the 
oócytes. In some way the energy of the body is applied 
to the completion of these important products, even at the 
expense of the remainder. 
If an individual containing young oócytes of small size je 
cut transversely near the middle of the body, and the pieces 
placed in clear water without food, both will usually live for 
months. The posterior piece does not regenerate, but the 
oócytes grow much more rapidly in it than in the anterior 
half, and may attain their full growth and be capable of 
fertilization and development before those contained in the 
anterior half have reached more than half or two-thirds of 
their full size. I can explain this fact only on the basis of 
the difference in activity of the two halves. The anterior 
half, possessing a head and brain, behaves like a normal 
animal, moving about actively, while the posterior half, being 
without a brain and never regenerating a head or brain, is 
much less active and less sensitive to stimuli. In other 
words, much of the potential energy of the body which is 
employed in the anterior piece in producing the various move- 
ments, and probably in a more intense general metabolism, 
is directed in the posterior piece to the completion of the 
