Xll PREFACE 



Many of the illustrations have been drawn specially for this book 

 by Miss B. Phillipson, to whose care and skill we wish here to 

 make acknowledgments. Particular thanks are due to Miss P. 

 Coombs for her help with typing and many other details of pre- 

 paring the book for press. Acknowledgment is hereby made to 

 those authors and publishers of the journals whose names appear 

 in the legends to the figures, by whose courtesy they are here 

 reproduced. 



We wish to express our thanks to Prof. E. S. Goodrich, F.R.S., 

 to Mr and Mrs R. Snow, and to Mr C. H. Waddington, who not 

 only read part or all of the manuscript but also made several helpful 

 suggestions. We are under a particular debt of gratitude to Dr Sven 

 Horstadius, and to Professor J. Runnstrom, who very kindly per- 

 mitted us to make use of some as yet unpublished results, and also 

 to Professor M. Hartmann who has kindly enabled us to reproduce 

 a figure from the (as yet unpublished) 2nd edition of his Allgemeine 

 Biologie. 



In conclusion, we should like to acknowledge our debt to the 

 late Dr J. W. Jenkinson, an Oxford man, and the pioneer of 

 Experimental Embryology in this country, and to express our deep 

 appreciation of the care and skill which the Cambridge University 

 Press has expended on the production of this volume. 



J.S.H. 

 G. R. DE B. 



January, 1934 



