Vi VERTEBRATE REPRODUCTIVE CYCLES 



zoologists. To all these my most grateful thanks are 

 offered. 



It is also a pleasant duty to recall the debts due to 

 Professor E. A. Spaul, who has criticized the manuscript, 

 to Mr. T. G. Onions, who has given invaluable assistance 

 throughout, to Dr. S. A. Peyton, the Librarian of the 

 University of Sheffield, who worked hard to obtain 

 copies of the more obscure references, and to Dr. L. D. 

 Brougersma of the Rijksmuseum in Leiden, who helped 

 with the scientific names listed at the end of the book. 



Finally, it is especially pleasant to have this oppor- 

 tunity to thank my wife for her constant help and 

 encouragement during the whole enterprise. 



w. s. bullough 

 Utrecht 



September y 1949 



PREFACE TO SECOND EDITION 



DURING the ten years that have elapsed since the 

 first printing of this book, research on the subject 

 of vertebrate sexual cycles has been active, especially in 

 the United States. An opportunity has now been taJcen 

 to revise the original text and to add an appendix that 

 will serve as an introduction to the more modern litera- 

 ture. 



In part, this revision is the outcom of a recent 

 Symposium on Photoperiodism held undei the auspices 

 of the American Association for the Advancement of 

 Science and the National Research Council and financed 

 by the National Science Foundation of the United States. 

 It was a privilege to be present on that occasion and 

 several of the new ideas now presented were clarified in 

 the debates that then took place. 



W. S. BULLOUGH 



