PHYSIOLOGY: LOEB AND NORTHROP 
383 
showed that the cancer cell is apparently immortal and the same could 
possibly be shown for many other tissue cells if it were only possible to 
mark them and thus follow them individually through a series of suc- 
cessive graftings. 
The experiments on hybridization also support the idea that natural 
death is due to some maladjustment in a compound organism. By 
fertilizing an egg with the sperm of another species we can bring about 
a shortening of the duration of life to an almost infinitely small fraction 
of the normal duration; and in some cases the opposite result may also 
be obtained. It depends whether in the hybrid the ma] adaptation of 
the parts is greater or smaller than in the pure breed. The problem 
then before the biologist is to find out the physico chemical character 
of this maladaptation which results in old age and death. 
2. In certain organisms life is divided into two or more well defined 
morphological periods separated by an equally well defined process, the 
metamorphosis. Thus in the frog the metamorphosis from the tadpole 
stage consists in the growth of the legs and the absorption of the tail 
and gills. We know now through Gudernatsch that this metamor- 
phosis can be brought about at any time by feeding the tadpole with 
thyroid, and it is possible that the natural duration of the tadpole 
stage is determined by the production of a certain quantity of a sub- 
stance, possibly a specific constituent or product of the thyroid gland 
in the tadpole. This substance acts on many different organs in the 
body in a different way, causing the buds of the legs to grow and caus- 
ing the tail and the gills to die by autolysis and by phagocytosis. 
This suggests the possibility that the termination of the second stage 
in the frog's life, namely natural death, is also determined by the pro- 
duction in the body of one (or more) substances which bring about the 
termination of the second stage of life as definitely as the thyroid substance 
terminates the tadpole stage. It matters little whether we call such 
substances terminating one of the stages in life hormones or poisons. 
3. In order to test the validity of such reasoning we have approached 
the problem by a method which one of us suggested in 1908^ for this 
purpose, namely by ascertaining whether there is a temperature coef- 
ficient for the duration of life. 
It is definitely established that a number of life phenomena have a 
temperature coefficient characteristic of chemical reactions, namely of 
about 2 for a difference of temperature of about 10°C.; although at 
the lower and upper temperature limit this coefficient changes more 
rapidly than it is usually found to change in purely chemical processes. 
This might be interpreted to mean that the order of magnitude of the 
