Experimental Study of Growth 251 
process could ever be maintained in equilibrium or reversed. 
Optimistic enthusiasts may claim that we are on the verge of 
this discovery, and that the day after to-morrow we may hope 
to begin to grow young; but we might well hesitate to take 
the prescription until we were certain that its action could be 
stopped, for it would be as awkward to disappear in an egg as to 
end one’s life in the other direction by growing old. Never- 
theless there is urgent need that the phenomena of growth 
be more thoroughly studied, and the work of such serious in- 
vestigators as Metschinkoff shows at least that there is a wide 
field for future investigation in the phenomena of senescence. 
Absorption of Parts by Larve 
We ordinarily think of all parts of the body growing old at the 
same time, but there are some apparent exceptions to this rule. 
An animal that is still growing may absorb certain parts of itself, 
and if the absorption of the part is not strictly comparable to the 
growing old of the whole organism, the two processes are so similar 
that they invite comparison. The most familiar example is the 
absorption of the tail of the tadpole, although similar cases are 
known to every embryologist. At the time of metamorphosis 
the large tail of the tadpole is rapidly absorbed, and although we 
have many descriptions of the way in which the breaking down 
and absorption of the tissues occur, we have not the slightest clew 
as to what initiates the process. We might imagine that with the 
changes in the gill region and with the imperfect beginning of 
lung respiration the amount of oxygen absorbed is decreased, 
so that the blood that reaches the tail can no longer supply that 
part with the necessary oxgyen, hence the tissues die and are 
eaten up by the phagocytes. The skin of the tail might absorb 
enough oxygen directly from the water or air to maintain itself, 
in part, during this time, hence is not so seriously affected. 
That the result is not due to this cause is shown by the fact 
that the gill respiration is maintained during the period of ab- 
sorption of the tail. It might be supposed that the beginning of 
lung respiration that takes place at this time might be connected 
