VI 



Preface 



should be avoided. Use should be made of tabulations 

 and graphs wherever possible. Since the book ad- 

 dresses itself mainly to active or potential investi- 

 gators (particularly in the experimental branches 

 of embryology, pathology, histology, endocrinology, 

 and developmental genetics), it would be of value 

 to point out gaps in our knowledge, the lack of 

 critical experimental data in unexplored or con- 

 troversial fields, and lines of research which would 

 deserve being followed up. In summary, the book 

 has as its major objective the synthesis and evalua- 

 tion of pertinent material selected from the whole 

 field of animal growth and development, with 

 emphasis upon recognized principles and mechan- 

 isms as well as on unsolved and new problems. 



With these suggestions we approached 

 twenty-five biologists prominent in the sub- 

 ject areas to be covered in the volume. They 

 readily accepted the invitation to collaborate 

 despite the tribulations and obligations in- 

 herent in such undertakings. The Editors are 

 very grateful to all of them not only for their 

 contributions to this book but also for the 

 spirit of cooperation and patience which they 

 exhibited during the years of arduous labor 

 that went into its preparation. As in all con- 

 certed creative efforts of this kind, progress in 

 realization was slow and at times faltering. 

 Contrary to the development of an organism, 

 no forces were at work to coordinate the sep- 

 arate creative efforts, and the Editors did not 

 see fit to weld the different contributions into 

 a uniform mold. Each contributor is finally 

 responsible for the organization, scope, and 

 content of his text. The Editors, on the other 



hand, must bear the responsibility for the 

 plan and the scope of the book, and assume the 

 blame for any defects in its structure. 



Whatever its imperfections and limitations, 

 the book represents a first-hand portrayal of 

 present-day views of animal development. As 

 such, we hope it may provide a basis of de- 

 parture for future endeavors of this kind. The 

 science of embryology, like the embryo, is 

 governed by the principles of progressive dif- 

 ferentiation, its present status only a transi- 

 tory moment between past and future — its 

 full potentialities yet to be realized. It is to the 

 pioneering spirit of those students who here- 

 after will enter the field of development and 

 growth that this volume is primarily dedi- 

 cated. In no lesser degree we inscribe these 

 pages to students and investigators in other 

 fields of the biological sciences, including 

 medicine and agriculture, who are constantly 

 confronted with problems of a developmental 

 nature and must deal with them. 



The Editors have been fortunate indeed in 

 the cordial relationship which has existed 

 between them and the publishers from the 

 beginning of this undertaking. We are most 

 grateful to them for their unlimited patience, 

 resourcefulness, and splendid cooperation in 

 making a book such as this all that it should 

 be in style and typography. 



B. H. WiLLIER 



Paul Weiss 

 Viktor Hamburger 



