viii PREFACE. 



altogether outweigh the theoretical objections to beginning 

 with the avian type. In many respects, it might be thought 

 desirable to commence with a holoblastic ovum ; but the 

 large food-yolk of the bird's egg is in many ways a great 

 assistance to the study of changes going on in the blasto- 

 derm. The chick is of all embryos the best to begin with ; 

 when its history has once been mastered, the subsequent 

 study of other forms becomes an easy matter. 



We venture to hope therefore that we shall meet with 

 general approval, in having described at considerable length 

 the history of the chick, and in hastening the publication of 

 our account, by bringing it forward in a separate form. 



In the earlier chapters, especially, we have gone into 

 very considerable detail ; and in order to make the account 

 intelligible to the beginner, have not been deterred by the 

 fear of wearying our readers with elementary and recapitu- 

 latory statements. Debated matters and details of minor 

 importance have been put in small print ; these may be 

 omitted by the student in reading the book for the first 

 time. Though we have sometimes introduced names in 

 connection with important observations, we have not thought 

 it necessary to do this systematically. For recent or debated 

 statements however, the authorities are always cited. 



The worth of such a book as this will be very small if 

 the student simply contents himself with reading what is 

 written ; and to facilitate the only really useful mode of 

 study, that of actual observation, a few practical instructions 

 have been added in an appendix. 



The readiness with which the development of the skull 

 can be studied in the chick renders it, in spite of obvious 



