DISCOVERY 



29 



details of shape and proportion. Later, Professor 

 Child, of Chicago, went further and showed that these 

 reduced worms were young, not only in appearance, 

 but in behaviour and physiological working too. 

 They were, in fact, young in all essentials, and if food 

 was given them, would once more grow up into 

 normal adults. By means of alternately feeding and 

 starving a single worm. Child was able to keep it 

 within prescribed limits of size and of physiological 

 age, while other individuals of the same brood had 

 given rise to eighteen generations — a period which, 

 translated into human terms, would take us back to 

 Chaucer's time. The process could apparently have 

 been indefinitely continued, but the Professor had 

 sufficiently proved his point, and turned to other 

 problems. 



So the Elixir Vita; has been found at last, and 

 revealed in startling simplicity as enforced periodic 

 starvation. Unfortunately, it is not a panacea, for it 

 does not work with any organism higher in the scale 

 than a flat-worm. . . . Ironical Fate ! 



There remains to be noticed another very curious 

 form of reversal, which is interesting on account of 

 the light it throws, in conjunction with the other 

 processes we have been considering, on happenings in 

 the history of the higher animals and of man himself. 



Perophora is the name given to an Ascidian nearly 

 related to Clavellina, but consisting of a series of very 

 small green individuals, springing at more or less 

 regular intervals from a creeping tube or stolon filled 

 with blood, much after the fashion of strawberry 

 plants along a runner. When this animal is placed in 

 unfavourable conditions, it begins to shrink in exactly 

 the same way as ClaveUina, but instead of becoming a 

 dense opaque ball, it grows smaller and smaller, and 

 finally ceases to exist at all, having been completely 

 resorbed into the stolon. 



When this curious process is investigated, it is found 

 that the various organs of the body become smaller, 

 owing to their component cells successively leaving 

 their places in the tissue and passing out as free units 

 into the blood-stream. It is as if a house were to 

 unbuild itself, the bricks flying out of their places in 

 the walls, and the rooms gradually shrinking until 

 nothing was left but a courtyard filled with loose 

 brick-heaps. 



Through all this elaborate unbuilding, the stolon 

 remains perfectly normal, and may even grow. It 

 would appear, therefore, that the individual proper is 

 more easily affected by adverse conditions than is 

 the stolon, and that this differential susceptibility, as 

 we may call it, leads to the reversal of development 

 in one part only of the whole organic system. 



This is borne out by the fact that when the same 

 system is kept in the best possible conditions, save 



for the fact that it is starved, it is the individual 

 which absorbs and feeds on the stolon. In brief, in 

 such a system, one part will be dominant ; and we 

 can give dominance to one or to the other by varying 

 the conditions. 



Differential susceptibihty is a universal phenomenon 

 in biology ; for, in fact, what we imply by the term 

 is simply that the parts of an organism are not all 

 alike in their sensitiveness to external agencies. 



When correlated with another fact in biology, how- 

 ever, it becomes of great significance. This other 

 fact is the existence of what have been called axial 

 gradients in the organisation of animals and plants, 

 persisting often throughout life, but alwaj-s existing 

 during development. 



By an axial gradient is meant a succession of parts 

 from one end to the other of the main axis of a living 

 organism, each part being successively a little less 

 active in its metabolism, its general chemical reactions, 

 than the one before. For instance, in a Planarian, or 

 again in an embryo mammal, it would appear that 

 the head is the most active region, the region where 

 metabolism is at its quickest, and that as we proceed 

 towards the tail, the rate of metabolism (using the 

 word in its most general sense) becomes less and less. 

 This conception again we owe to Professor Child. Into 

 its details we cannot here enter ; it must suffice to 

 look at some of its consequences. 



Other things being equal, we would expect that a 

 piece of machinery which is working quickty will be 

 more damaged when an iron bar, say, is thrust into 

 its midst than if it were working slowly. The analogy 

 seems to hold for organisms. Poisons, whether nar- 

 cotics, or those that stop oxidation, or acids, or 

 mercury salts, when administered in such great dilu- 

 tion as not to kill but only to damage, act first and 

 most strongly on the more active parts of the organism. 

 Or if their strength is such as to kill after several 

 hours only, the more active parts will die first, and 

 death will creep down the axial gradient. 



It follows that we should be able to find a dilution 

 of poison which will affect the more active parts alone, 

 and leave the less active parts to all intents and pur- 

 poses undamaged. 



This has been done. If, for instance, when a 

 developing egg of a marine worm such as Cha;topterus 

 is placed in a very dilute potassium cyanide solution, 

 the apical region of the larva, with its little sense- 

 organ and tuft of cilia, is acted upon and fails to grow 

 to the normal size. The hind-region of the body also, 

 which is preparing for active growth into the trunk 

 of the worm, and is found to start dying as quickly as 

 the " head " region in dilute poisons, is reduced in the 

 same way. The middle region of the body, however, 

 is not affected ; and since there is a definite amount 



