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[OcroseR 5, 1916 
cesses which lead to death in the end are the 
conditions and processes which make man what 
he is” (p. 310). 
It might seem that the author’s thesis was here 
petering out, but the next step is important. Prof. 
Child does not believe much in.the “segregation 
of the germ-plasm ” ; he thinks that the germ-cells, 
like the somatic cells, exhibit progressive differ- 
entiation and senescence, that ‘‘the fully developed 
gametes are physiologically old, highly differ- 
entiated cells, which are rapidly approaching 
death and in most cases actually do die soon after 
maturity unless fertilisation occurs” (p. 357). 
They are of advanced physiological age and low 
metabolic rate. They must undergo de-differentia- 
tion and rejuvenescence before they can enter upon 
a new period of development. The parthenogenetic 
egg has not lost its capacity of reacting to isola- 
tion, but ordinary gametes can undergo de-differ- 
entiation and reconstitution only after fertilisation. 
In this ingenious way, which cannot be justly 
represented in brief statement, the author inter- 
prets gamete formation as a means of securing 
rejuvenescence. Reproduction is always the re- 
constitution of a new organism from a part of 
one previously existing, but the author stands for 
the new idea that rejuvenescence occurs during 
early development, and the zoological evidence 
that he adduces is very interesting. “The 
organism, when it begins its active independent 
life at the end of the embryonic period, is certainly 
very much younger in every respect than the 
gametes before fertilisation” (p. 403). It has 
become younger as it has grown older. This 
seems a little over-ingenious, but the general idea 
that the early development means very literally 
making a fresh start becomes concrete and vivid 
in the author’s exposition. 
Let us attempt a more compact statement of 
the author’s interpretation of age-changes. The 
numerous theories of senescence fall into two 
groups—those which regard it as an incidental 
imperfection (due to wear and tear, incomplete 
elimination of waste products, and so on), and 
those which regard it as an inevitable feature of 
development. Prof. Child’s experiments point to 
the second view. After the earliest stages of de- 
velopment there is a progressive change in the 
direction of greater physiological stability in con- 
sequence of changes in the substratum and addi- 
tions to it in the course of growth and differentia- 
tion. As the proportion of relatively stable con- 
stituents in the substratum increases, there is a 
decrease in the metabolic activity of each unit of 
weight or volume of the organism. Thus cells 
may become loaded with non-protoplasmic enclo- 
sures, and skeletal or supporting tissues arise. It 
is also probable that the increasing density and 
aggregation of the colloid substratum may lead to 
an actual decrease in the rate of chemical re- 
actions, and that the increase in density and thick- 
ness and the decrease in the permeability of mem- 
branes may retard the exchange through them. 
Thus and thus does ageing begin while the life is 
still young. ‘‘The decrease in rate of metabolism 
is a part of development itself, and not an acciden- 
NO. 2449, VOL. 98] 
tal or incidental feature of life. The decrease in 
metabolic rate during development is in fact a 
necessary and inevitable consequence of the asso- 
ciation of the chemical reactions which constitute — 
metabolism with a colloid substratum produced by 
the reactions ” (p. 184). But there is another side 
to all this, which has been for the most part over- 
looked because it is inconspicuous in higher forms. 
There are processes of reduction as contrasted 
with growth, of retrogressive as contrasted with 
sub- | 
progressive development. Accumulated 
stances and structures may be broken down, and 
the self-fettered metabolism increases in rate. 
There is a retrogression towards the embryonic 
condition, as is familiarly seen in the pupa of a 
fly. “Dynamically rejuvenescence consists in 
increase in rate of metabolism, and morphologi- 
cally in the changes in the substratum which per- 
mit increase in rate.” And this rejuvenescence is 
as essential a feature of life as senescence. Age- 
ing has been too much regarded as “a rather 
mysterious process, quite different from anything 
else in the life cycle,” but it is simply a conspicu- 
ous expression of what occurs in minor rhythms 
continually. Thus the period of “loading” of a 
gland-cell is a period of decreasing metabolic 
activity (of ‘“‘senescence”), and the period of dis- 
charge one of increasing activity (of “rejuvenes- 
cence”), which makes possible a repetition of the 
cycle. So there are alternations of fatigue and 
recovery, of quiescence and activity, and ‘“‘ whether 
we call one cycle an age-cycle and another some- 
thing else is of little importance, except as regards 
convenience.” It is further suggested that there 
may be secular senescence and rejuvenescence in 
racial evolution. ‘‘The age-changes in the organ- 
ism are merely one aspect of Werden und Ver- 
gehen, the becoming and passing away, which 
make up the history of the universe.” 
It is out of the question to express in a few lines 
more than an appreciation of a carefully executed 
piece of work of this magnitude. It is rich in sug- 
gestiveness and original ideas, and gives us a new 
view of the organism and its vital tides. Much 
depends on the soundness of the susceptibility 
method of determining the rate of metabolism, for 
a great deal of the evidence relies on this. We 
confess to a feeling that the superstructure of 
interpretation is too broad for the Planarian basis 
on which it mainly rests. Time is required for a 
consideration of the evidence given of rejuvenes- 
cence in the early stages of development in higher 
animals, and for weighing the author’s reasons 
for rejecting Weismann’s conception of the apart- 
ness of the germ-plasm. But it is a great satis- 
faction to meet with such a fine instance of reso- 
lute biological thinking, and we offer Prof. Child 
our congratulations. 
(2) The author’s main contribution to the. prob- 
lem of organic individuation is the demonstration 
of a distinct gradient in the rate of metabolic reac- 
tions along the chief axis of various axiate types. 
The apical or head region is primarily the region 
of highest rate of metabolism, and, in general, 
regions nearer to it have a higher rate than 
regions farther away. Moreover, in experimental 
~_— os 
