Acknowledgments 



The author expresses his great obUgation to and appreciation for the supe- 

 rior artistic abilities, continual patience, and conscientious effort of Elisabeth 

 R. Swain who executed the difficult task of preparing — with certain exceptions 

 — the illustrations for this text. He also wishes to express his sincere thanks 

 to Edna R. White and Julia A. Lloyd who contributed illustrations. These 

 three artists were most exact in carrying out the author's instructions for illus- 

 trations, and also in transforming his preliminary sketches into finished draw- 

 ings. 



The author is indebted greatly to Wistar Institute of Anatomy and Biology, 

 Philadelphia, for permission to redraw various illustrations from the journals 

 published by the Wistar Institute. Appreciation similarly goes to the Carnegie 

 Institution of Washington; The Marine Biological Laboratory, Woods Hole, 

 Mass.; Williams and Wilkins Co., Baltimore; University of Chicago Press, 

 Chicago; Yale University Press, New Haven; Academic Press, Inc., New 

 York; Museum of Comparative Zoology at Harvard College; Oxford Uni- 

 versity Press, Inc., New York; Ginn and Co., Boston; W. B. Saunders Co., 

 Philadelphia; McGraw-Hill Book Co., Inc., New York; Henry Holt and Co., 

 Inc., New York; W. W. Norton and Co., Inc., New York; John Wiley and 

 Sons, Inc., New York; J. B. Lippincott Co., Philadelphia; The Macmillan Co., 

 New York and London; Knopf, Inc., New York; Appleton-Century Co., 

 Inc., New York; Sidgewick and Jackson, Ltd., London; Cambridge Univer- 

 sity Press, England; and Columbia University Press, New York. 



To his colleagues in the Department of Zoology of the University of Penn- 

 sylvania the author owes a debt of appreciation for encouragement during the 

 writing of the manuscript, especially to Dr. J. Percy Moore, Dr. D. H. 

 Wenrich, and Dr. L. V. Heilbrunn. Acknowledgments and appreciation go 

 to Mrs. Anna R. Whiting, also of the Department of Zoology, and to Dr. 

 Miles D. McCarthy of the Harrison Department of Surgical Research of the 

 University of Pennsylvania Medical School and the Department of Zoology, 

 Pomona College, Claremont, California, who read much of the manuscript and 

 offered valuable suggestions. Frances R. Houston, Librarian of the University 

 of Pennsylvania Medical School, and Elizabeth D. Thorp, Librarian of the 

 Botany-Zoological Library of the University of Pennsylvania, deserve sincere 

 thanks for cooperative understanding and help in securing and placing many 

 periodicals at the author's disposal. Various students contributed clerical efforts 

 toward the completion of this work, especially Barbara Neely Gilford, Carolyn 

 Kerr, and Louise Mertz. Their endeavors are appreciated greatly. 



