62 : EDWIN G. CONKLIN 
maturation, fertilization and cleavage of the egg. Loeb first 
showed that the immature egg, with germinal vesicle intact, is 
metabolically inactive; it absorbs but little oxygen and gives off 
little carbon dioxide. On the other hand when the membrane 
of the germinal vesicle dissolves, metabolic activity increases, 
and unless the egg is started in the process of development, by 
fertilization or other means, it soon dies. Lyon (’04) found that 
during the cleavage of the sea urchin egg the evolution of carbon 
dioxide is more rapid during the periods of division than during 
those of rest. Warburg (’08) found that the fertilized sea-urchin 
ege uses six to seven times as much oxygen as the unfertilized 
egg. It is well known that the condensed chromatin of the chro- 
mosomes is brought into intimate relation with the protoplasm 
during mitosis, and of course the same is true of the condensed 
chromatin of the sperm head following fertilization. We may 
conclude, I think, that mitosis increases metabolism by facili- 
tating the interchange between nucleus and protoplasm, and 
particularly by setting free chromatin in the protoplasm, either by 
the dissolution of the nuclear membrane, or by the introduction 
of the sperm head in fetilization. 
Rapid and intimate interchange between the chromatin and 
the protoplasm is the condition of rapid metabolism, and ex 
hypothese of rejuvenescence; slow interchange is the condition of 
slow metabolism, and of senescence. Such a view has many 
points in common with the hypotheses of Minot and Hertwig, 
while it avoids many of the serious difficulties which those hypoth- 
eses encounter. It is thus evident that one may hold, with 
Minot and Hertwig, that the germ cells before maturation are 
senescent, and that maturation, fertilization and cleavage rep- 
resent a rejuvenescence, without necessarily connecting these 
processes with the nucleus-plasma ratio. 
3. Lillie (1910) holds that this is due to increased permeability of the plasma 
membrane during division. 
