PREFACE. vii 



and I have purposely emphasised the more important of these 

 gaps in the hope of drawing to them the attention of those 

 who may have opportunity of filling up the deficiencies. The 

 bibliographical lists at the ends of the several chapters have 

 been deliberately curtailed, and include only those books and 

 papers which appear to me of real importance : my object in 

 this, as indeed in all respects, has been to produce a book which 

 shall be useful rather than encyclopaedic. 



I have, in the text, made no attempt to assign the several 

 statements to their original authors : to do so would have bur- 

 dened the book unduly. It will be well, however, to give here 

 the main sources from which the facts are gathered, in order 

 that- 1 should not receive credit which is really due to others. 



In the chapter on Amphioxus I have had to rely entirely on 

 the work of other observers. The descriptions of the earlier 

 stages are from the well-known accounts by Kowalevsky and by 

 Hatschek : for the later stages I have depended mainly on the 

 recent researches of Professor Lankester and Mr. Willey. 



Except as regards the processes of maturation and fertilisa- 

 tion of the egg, which are described from Oskar Schultze's 

 papers, the chapter on the Frog is based almost entirely on 

 my own observations, supplemented by the work of some of my 

 pupils. 



The development of the Chick has been described more often 

 than that of any other animal. I have, however, worked over 

 the greater part of the ground again, with special reference to 

 this book. I have derived much assistance from the researches 

 of Duval, especially in regard to the earlier stages of develop- 

 ment. 



I have not myself studied the-processes of segmentation, and 

 formation of . the blastodermic vesicle in the Rabbit ; but I have 

 had the advantage of examining a very excellent series of pre- 

 parations by my friend Mr. Assheton. In my descriptions of 

 these earlier stages I have relied mainly on the accounts of 

 E. van Beneden, and of Kolliker. The later stages, from the 

 first appearance of the embryo onwards, I have studied in con- 



