PREFACE TO THE THIRD EDITION 



In the present edition the general plan of the book, as outlined in the 

 preface to the first edition, remains unchanged with the exception that 

 Practical Suggestions at the end of each chapter and the Appendix dealing 

 with general technic are omitted. Certain changes have been made in the 

 text and illustrations. Several chapters have been rewritten in the light of 

 recent studies, numerous changes have been made throughout the book in 

 view of the advances made in the science, and a number of new illustrations 

 supplant the old. 



The writers wish to thank Mr. Adolph Elwyn for the revision of the. 

 chapters on Maturation and Fertilization. 



THE AUTHORS. 

 JULY 24, 1916. 



iii 



81530 



