METAMORPHOSES OF SCEPTICISM 



Ovists believed that mammals developed from the egg alone, and that 

 the spermatozoa were adventitious worm-like organisms, perhaps 

 parasites, as the termination "-zoa" of their modern name still indicates 

 to-day. The Animal culists, on the other hand, believed that the 

 animal originated from a spermatozoon only, and that the egg was if 

 anything only a kind of box in which it could develop. The phenomena 

 of inheritance alone should have sufficed to indicate that this deadlock 

 was an absurdity, but it took nearly two centuries before the functions 

 of egg and spermatozoon were understood, and the contradiction was 

 resolved. In the later history of embryology we find a similar contra- 

 diction. When the study of the fate of parts of the egg began, it was 

 at first believed that all development is mosaic, i.e. that any injury 

 to the egg at the beginning of development is reflected in a corre- 

 sponding injury in the finished embryo. Then came the discovery that 

 in some eggs, at any rate, regulative development can occur, i.e. that 

 from a half-egg, or even a quarter-egg, a normal embryo, though 

 small, can be formed. These facts were made the basis of vitalistic 

 theories, but the sharp contradiction was at length resolved by the 

 finding that the eggs of some species are mosaic and that those of 

 others are regulative, the only difiference between them being the 

 exact time at which the determination of the fates of the various 

 parts occurs. 



The same syntheses of contradictions are going on in biochemistry. 

 Some thirty years ago lactic acid was thought to be the causative 

 agent in the contraction of muscle. After the discovery of phosphagen, 

 this substance in turn was regarded as the most important. We know 

 now that neither of these substances is connected with the final 

 process whereby energy is transferred from chemical processes to the 

 muscle fibre but another substance altogether, adenylpyrophosphate. 

 Phosphagen is one of the substances involved in the cycles of phos- 

 phorylation by which energy is transferred, while the lactic acid is 

 simply the waste-product of the breakdown of glycogen whereby 

 the chemical energy is provided. Or again, adrenalin has long been 

 known to contract blood-vessels and also raise the blood sugar by 

 mobilising liver glycogen. This latter action was thought to be due 

 to the constriction of the liver's small blood-vessels, with consequent 

 anoxaemia and asphyxia. But it was then found that even in well- 

 oxygenated liver cells, adrenalin causes a breakdown of glycogen. This 

 contradiction was subsequently resolved by the further finding that 

 adrenalin intervenes in the oxidative synthesis of glycogen in the cell. 



17 



B 



