ACKNOWLEDGMENTS 



The author desires to acknowledge his gratitude to his 

 colleague, Dr. Leona Zacharias; to his secretary, Miss 

 Dorothy Phillips; to his assistant, Mrs. William Seitz, for 

 their aid in the preparation of the manuscript and in proof- 

 reading; and to Mr. Sidney Shapiro who prepared many of 

 the illustrations. He gratefully acknowledges his thanks to 

 Dr. Selig Hecht, Dr. W. J. Crozier, and Dr. Gregory Pincus 

 of the editorial board of the Experimental Biology Mono- 

 graph Series for reading the manuscript and for their helpful 

 suggestions and constructive criticisms. 



Acknowledgment is also given for the kind permission to 

 reproduce other authors' figures from the following sources : — 



Chicago University Press, Chicago, Illinois 

 J. & A. Churchill, Ltd., London, England 

 Williams & Wilkins Company, Baltimore, Maryland 

 Cambridge University Press, The Macmillan Company, 



New York 

 Rockefeller Institute for Medical Research, New York 

 The C. V. Mosby Company, St. Louis, Missouri 

 American Medical Association, Chicago, Illinois 

 Wistar Institute of Anatomy & Biology, Philadelphia, 



Pennsylvania 

 Paul B. Hoeber, Inc., New York 



