vi PREFACE 



ally avoided at times the discussion of the more theoretical 

 problems arising from the experiment, for it seems to me that 

 such discussions are out of place in a volume of this sort. Only 

 the early stages of the development have been considered, 

 because almost all of the experimental work on the frog's egg 

 has been done on the early stages, and also because I am more 

 familiar with the development and with the experiments of this 

 period. Moreover, the later stages have been recently most 

 admirably described by Marshall in his Vertebrate Embryology. 



A few words of personal explanation may be added. For 

 several years I have been collecting the material for the present 

 volume, but as the literature is so extensive and as I have had 

 other work to do first, I made but slow progress. In the 

 summer of 1893 I set seriously to work, and owe much to the 

 admirable facilities offered by the University of Berlin. I take 

 pleasure in acknowledging my indebtedness to Geheimrath 

 Professor Fr. E. Schulze for many privileges and kindnesses 

 extended to me in Berlin. The work was continued irregu- 

 larly during the winter of 1893-1894 while enjoying the oppor- 

 tunities of the Stazione Zoologica in Naples. During the 

 winter of 1894-1895 the material was brought together and in 

 the summer of 1896 at Zurich the manuscript was almost com- 

 pleted. I gladly take this opportunity to thank Professor 

 Arnold Lang for many courtesies extended to me during two 

 visits to Zurich. Dr. Driesch has most kindly looked over 

 some of the chapters, and has made many valuable sugges- 

 tions. Dr. H. H. Field has also examined a part of the 

 manuscript and helped me in several directions. To Professor 

 E. B. Wilson I am under heavy obligations, and owe much 

 to his valuable suggestions and corrections. To Dr. H. 

 Randolph I owe a debt of gratitude for kindly advice and 

 criticism. I am also greatly indebted to Professor Joseph W. 

 Warren and to Professor E. A. Andrews for advice in con- 

 nection with the revision of the proof. 



