Acknowledgements 



My thanks for the use of copyright illustrations are due to authors 

 and editors of works and periodicals mentioned in the legends and 

 in the References, and to the following proprietors and publishers: 



Academic Press Inc., New York; American Association for the 

 Advancement of Science, Washington, D.C.; American Institute 

 of Biological Sciences, Washington, D.C.; American Physiological 

 Society, Bethesda, Md.; American Scientist, Princeton, New Jersey; 

 Chatto & Windus, Ltd., London; The Journal of Physiology, 

 Cambridge (England); Long Island Biological Association, Long 

 Island, New York; Macmillan (Journals) Ltd., London; Per- 

 gamon Press Ltd., Oxford; Quarterly Review of Biology, State 

 University of New York; The Rockefeller University Press, New 

 York; The Royal Society, London; W. B. Saunders Company, 

 Philadelphia, Pa.; The Wistar Institute of Anatomy and Biology, 

 Philadelphia, Pa. 



Grateful thanks are due to Dr H. W. Lissman for reading the 

 manuscript and offering invaluable advice and critical comments. 

 I would especially like to thank Dr J. E. Treherne for his many 

 helpful suggestions on the manuscript. Much of the book was 

 written while I was a visitor in the Zoological Laboratory, Uni- 

 versity of Cambridge, and I thank both the staff of the laboratory, 

 who helped in reproducing the illustrations, and the John Simon 

 Guggenheim Memorial Foundation, whose generous financial 

 support made my visit possible. 



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