iv PREFACE 



of embryology is a careful first-hand study of some one species. 

 It is in this sense that the book may serve as an introduction to 

 embryology, if its study is accompanied by careful laboratory 

 work. In some respects it is fuller, and in others less complete, 

 than other books with which it might be compared. On its 

 comparative and experimental sides, embryology is the only key 

 to the solution of some of the most fundamental problems of 

 biology. The fact that comparative and experimental embry- 

 ology receive bare mention is not due to any lack of appreciation 

 of their interest and importance, but to the conviction that the 

 beginner is not prepared to appreciate these problems at the 

 start; to the belief that our teachers of embryology are com- 

 petent to remedy omissions; and finally to the circumstance 

 that no one book can, as a matter of fact, cover the entire field, 

 except in the most superficial way. 



The development before laying and the first three days of 

 incubation are treated by stages as far as possible, and this mat- 

 ter constitutes Part I of the book. It involves the study of the 

 origin of the primordia of most of the organs. The matter 

 concerning the later development is classified by the organs 

 concerned, which seems to be the only possible way, and this 

 constitutes Part 11. The first part is complete in itself, so far 

 as it goes, and no doubt it will be the only part consulted by 

 some students. 



The attempt to present a consecutive account of the develop- 

 ment of the form on which so many classics in the history of 

 embryology have been based is no slight undertaking. The 

 author can hardly hope that he has avoided omissions and errors, 

 and he will be sincerely grateful to those who call such to his 

 attention. 



