PREFACE 



I 



General embryology should occupy an important place in the 

 collegiate study of biology. In no other connection are the 

 essential phenomena of life better illustrated, in no other form 

 are they more readily appreciated. The facts of embryology 

 lead directly to the great problems of the science of biology as 

 it exists to-day, and many fundamental biological conceptions 

 either are directly connected with, or are illuminated by, the 

 study of the early phenomena of individual development. 



The author's experience has clearly indicated that the subject 

 has this value as a collegiate study. Indeed, the book is the 

 direct outgrowth of such experience, and it has, in substance, 

 been in use as such a text for several years. In its present form 

 it is hoped that it will be found useful to the student who is 

 endeavoring to comprehend the general principles of the science 

 of life, as well as to the student preparing for the professional 

 study of some field of biology or of medicine. 



Its design as a textbook, rather than as a handbook, accounts 

 for certain characteristics. The topics considered have through- 

 out been approached from the standpoint of their general 

 biological relations, and in the selection of the facts mentioned 

 and the topics discussed, as well as in the style and method of 

 presentation, the student has been first in mind. The arrange- 

 ment of the subject matter in two sizes of type may prove 

 useful for those undertaking a brief course. In a few instances 

 this has involved slight repetition, but repetition is not always 

 a pedagogic evil. 



At the end of each chapter will be found a list of references to 

 literature. Usefulness to the student has been the only 

 criterion in determining the admission of titles to these lists. 

 Consequently there will be found titles of works of historical 

 importance, of recent works containing contributions of impor- 



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