a modern systhesis of our knowledge of the principles and mechanisms of deve- 

 lopment. The value of such a work with respect to the trends of overspecial- 

 ization of scientists at the present time does not need any further argument or 

 emphasis. 



Since such a project far exceeds the abilities of any one individual, the work 

 was planned by the editors in 1947 as a collaborative work, thus sacrificing a 

 part of the desired unity and integration. The individual contributors were 

 asked to describe the principles upon which the developmental processes are 

 founded on the basis of crucial and analytically strong experiments, and to 

 indicate still unexplored or controversial as well as desirable new lines of ana- 

 lysis. We must confess that some contributions better fulfill all these require- 

 ments than do others. 



The editors have given themselves an enormous task, perhaps even a task 

 which is in part impracticable. The field of research treated is not only very 

 wide, but is a rapidly extending field. Our views upon many aspects of it. and 

 upon many problems is changing so rapidly that such a work can only intend 

 to be a momentary survey of our knowledge and understanding, a survey 

 which should anyhow be up to date. It is particularly this last aspect which is 

 impracticable in a collaborative work of this size. Several contributions were 

 already completed several years ago. and could unfortunately not be brought 

 entirely up to date. Notwithstanding these partially unavoidable but neverthe- 

 less serious objections, the general scope of this book and its internal organizat- 

 ion demand our great admiration and gratitude to the editors for their initiat- 

 ive, and for the themendous work done by the editors and collaborators. The 

 work forms an excellent guide into the many-sided aspects of developmental 

 analysis and into the enormous literature of this field. 



P. D. NIEUWKOOP 



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