114 SECTIONAL ADDRESSES. 
animals of ponds and streams, the Planarians, the act of procreation can be 
both naturally and artificially checked, and a return to a less highly organ- 
ised state can be induced. In a similar way the act of sterilization induces 
fresh vigour in some of the higher animals. Finally, in many animals the 
body undergoes periodic retrograde evolution, renews its youth, returning 
to an undifferentiated state in which it passes the winter with heightened 
powers of resistance, and on the advent of spring redevelops its 
organisation. 
Evidence for the Hypothesis of Metabolic Gradients. 
(A) Aazal susceptibility. 
The evidence for these far-reaching conclusions as to the nature of the 
living organism is partly direct and experimental, and partly indirect and 
observational. The direct evidence has been drawn from experiments 
by Professor Child and his school on Protozoa, Coelenterata, Planarians, 
Liver-flukes, Annelids, Echinoderms, Fishes and Amphibia extending 
over about fifteen years. Recently Dr. Shearer (5) has repeated these 
experiments on the chick and on earthworms, with results entirely confirm- 
ing the conclusions of Child and his pupils. A critical review of the evidence 
has recently been published by Child and Bellamy (5a). 
The first class of evidence relates to axial susceptibility to the action of 
toxic or narcotic substances. When immersed in, for example, a weak 
solution (0.001 mol) of potassium cyanide in well-water, the ‘ head-end ’ 
of the whole animal or the apical pole of the egg is the first portion of the 
body to undergo disintegration, and this is followed by a succession of 
stages during which the process slowly spreads downwards. In general, 
the susceptibility-curve plotted on the basis of time-ordinates against these 
stages as abscissae, shows a much sharper fall for young than for older 
animals of the same species if the solutions are above a certain degree of 
concentration. If very dilute solutions are used, the opposite result is 
obtained. Immunity is gained more rapidly by the young than by the old. 
These results may be explained as due to the action of the cyanide on the 
oxidation-process and possibly also on the physical character of the 
colloidal protoplasm. The important point is the definite relation of 
disintegration to the animal’s axis. The ‘ head-region’ or the apex of the 
egg disintegrates first and the basal region last. The evidence therefore 
tends to show that the susceptibility gradient is evidence of the existence 
of a metabolic gradient. 
Estimations of this kind have been made by the use of a large number 
of narcotics and poisons and the results have been confirmatory. More 
recently, other methods of testing the presence, course, and strength of 
these gradients have been devised. Dr. Tashiro (6), for example, has applied 
to the nerves of the body an exceedingly delicate test (the Tashiro 
biometer) for the estimation of carbon dioxide in minimal quantities, and 
has shown that a gradient exists following the direction of the impulse 
along the nerve. Again, Child himself, and later Shearer, have demon- 
strated the presence of axial gradients in starfish and chick respectively, 
by the use of acetone and other substances, which are precipitated in the 
tissues of the living developing animal by oxidation, thus giving an ocular 
demonstration of the track of the primary gradient. Unquestionably 
