X PREFACE 



this, there are always two levels of approach. One of these is 

 broadly biological, while the other is physiological in the stricter 

 sense. The prime aim of the worker approaching the problem on 

 the physiological level will always be to analyse the processes in- 

 volved in terms of physics and chemistry. The worker on the 

 biological level will aim at discovering general rules and laws which 

 he is content to leave to his physiological colleague for future 

 analysis in more fundamental terms, but which, meanwhile, will 

 give coherence and a first degree of scientific explanation to his 

 facts. Both methods are necessary for progress; and while most 

 biologists hope and expect that one day their laws will, thanks to 

 the labours of their physiological colleagues, be made compre- 

 hensible in the most fundamental physico-chemical terms, they 

 can reflect that it is they who must first reveal the existence of these 

 laws before the pure physiologist can hope to begin his analysis. 

 The biologist can also remember that these laws have their own 

 validity on their own level, whether they be physico-chemically 

 analysed or not. 



We may take a salient example from the contents of this book. 

 Spemann's discovery of '' organisers " in the process of gastrulation 

 of Amphibia, and the extension of the concept to other stages of 

 development and to other groups of organisms, have made it 

 possible to understand on the biological level many processes of 

 development which were previously obscure. At the moment we 

 can only throw out crude guesses as to the underlying physiology 

 of organisers and their eflFects, but the discovery opens a new field 

 of research to physiologists, which they themselves would not have 

 been likely to hit upon for many years. And even if and when the 

 physiological analysis has been made, the empirical biological laws 

 concerning organisers will not lose their validity or their interest ; 

 they will merely have been extended and deepened. 



At the present moment, research into developmental problems 

 is being actively prosecuted on both the biological and the physio- 

 logical levels. Following up the early work of Roux, Hertwig, 

 Driesch, Herbst, Jenkinson, Delage, Brachet, Morgan, and Wilson, 

 a flourishing school of Entwicklimgsmechanik has grown up in 

 Germany, and another, no less successful, in the United States. 

 Meanwhile, on the physiological side, the advance has also been 



