lv 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 behef 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 II. 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. 
