5 a a ee - ee 
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URE 85 
THURSDAY, O€TQBER 5, 1916. 
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THE ORGANISM AS PHG:NIX. 
(1) Senescence and Rejuvenescence. By C. M. 
Child. Pp. xi+481. (Chicago: University of 
. Chicago Press; London: Cambridge University 
Press, 1915.) Price 16s. net. 
(2) Individuality in Organisms. By C. M. Child. 
Pp. x+213. (Chicago: University of Chicago 
Press; London: Cambridge University Press, 
1915.) Price 5s. net. 
(1) Ts this able study of senescence and re- 
juvenescence Prof. Child gives us the 
fruits of fifteen years of research on the age- 
changes of the lower animals. Much of the book 
is a record of observations and experiments which 
have not been previously published; the rest is a 
thoughtful working-out of a new conception of the 
organism, which must be taken account of by 
all biologists. The most prominent new feature 
is the author’s attempt to show that “in the 
organic world in general rejuvenescence is just as 
fundamental and important a process as senes- 
cence.” In the higher forms the possibilities of 
rejuvenescence seem to be very narrowly limited, 
and may be scarcely recognisable save in con- 
nection with sexual reproduction; but in the 
simpler organisms, such as the Planarian worms 
on which the author has published many re- 
searches, it is a characteristic feature of life. 
In seeking to determine the degree of ageing 
in an organism we are accustomed to look out 
for certain structural changes familiar in man 
and mammals, or we measure the decreasing rate 
of metabolism per unit of substance. For lower 
animals, however, other tests must be used; thus 
advantage has been taken of the general relation 
between metabolic condition and susceptibility or 
resistance to cyanides or other narcotics. With 
differences in susceptibility there also correspond 
differences in carbon. dioxide production as 
estimated by Tashiro’s beautifully delicate “bio- 
meter.” In other cases the susceptibility or resist- 
ance of the organism to: the depressing agent 
may be determined indirectly by the creature’s 
ability to become acclimated to a given concen- 
tration. 
Numerous experiments on Planarians show that 
the rate of metabolism, measured by the suscepti- 
bility method, decreases with advancing age, and 
it is concluded that.a decrease in rate of meta- 
bolism is at least very generally associated with 
growth and differentiation. An isolated fragment 
of a Planarian begins by developing a new head 
and tail, and then undergoes an extensive re- 
organisation into a new individual of small size. 
Some parts atrophy and disappear; new parts arise 
and differentiate; the development in reconstitu- 
tion is not essentially different from embryonic 
development. All this and much more is familiar, 
but the novel point is that the reconstituting 
Planarians are physiologically vounger than those 
from which the pieces came. As measured by the 
NO. 2449, VOL. 98] 
susceptibility method they exhibit rejuvenescence, 
and “the degree of rejuvenescence is in general 
proportional to the degree of reorganisation in 
the process of reconstitution of the piece into a 
whole ’’ (p. 118). 
The next step in the experimental argument is 
very interesting: it is the demonstration (by the 
susceptibility method) that pieces spontaneously 
isolated in Ptanarians and Hydroids are physio- 
logically younger than the individuals ffom which 
they separate. In other words, rejuvenescence is 
associated with asexual or agamic reproduction. 
Furthermore, a Planarian starved for months lives 
on its own resources and becomes reduced in size. 
Some cells disappear and others become small. 
This is well known, but the new fact is that the 
starving animals undergo rejuvenescence, the 
degree varying with the degree of reduction. If 
the susceptibility method is trustworthy. they be- 
come physiologically younger, and this conclusion 
was corroborated by estimating the production of 
carbon dioxide. There is here a strong biological 
argument for asceticism. The starveling is 
brought back from an advanced age to the begin- 
ning of post-embryonic life; it is almost reborn. 
“The metabolic current is forced to erode its 
channel instead of depositing material along its 
course” (p. 179). Moreover, with certain foods 
a progressive senescence from generation to 
generation occurs, while with other foods senes- 
cence and rejuvenescence seem to balance in each 
cycle. 
In plants and simple animals, such as polyps, 
asexual reproduction is frequent, and this brings 
with it some measure of rejuvenescence. ‘Often 
the decrease in metabolic rate with advancing 
senescence is the primary factor in bringing about 
physiological isolation of parts, reproduction, and 
rejuvenescence ” (p. 260), and thus senescence 
may be automatically compensated for. Of great 
interest, also, are various processes of cellular de- 
differentiation which occur in both plants and 
lower animals, and this also will act as a retarda- 
tion of senescence. In the higher animals the 
later atrophic stages of senescence are con- 
spicuous, while in the lower forms they either do 
not appear or else occur in only a few cells at 
any given time. ‘The apparent continuity and 
irregressibility of senescence in man and the 
higher forms is responsible for the very general 
belief that the process is irregressible everywhere, 
but the plants and lower animals. show us clearly 
enough that this is not the case” (p. 289). In the 
higher reaches senescence is less frequently inter- 
rupted, but it is essentially the same throughout, 
and occurs “wherever the progressive changes are 
not balanced or overbalanced, by regression.” 
With senescence in man and domesticated mam- 
mals everyone is familiar, but what of the alleged 
rejuvenescence? The author answers that some 
degree of rejuvenescence occurs in certain tissues, 
but that on the whole it is grievously limited by 
the greater stability of the substratum. ‘For his 
high degree of individuation man pays the penalty 
of individual death, and the conditions and pro- 
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