INTRODUCTION. 7 



the manner in which the embryo is formed, and the way in 

 which the rudiments of the most important organs of the 

 chick arise. We shall follow a chronological order, tracing out 

 the changes day by da} 7 (or with even shorter periods), during 

 the first few days. We are convinced that this method 

 (adopted by Von Baer) is on the whole the one which most 

 commends itself to the learner. It has of course its disad- 

 vantages ; and in several instances we have found it desirable 

 when describing, at its appropriate date, the most striking 

 phase in the development of an organ, at once to follow up 

 the subsequent history, instead of giving it piecemeal after- 

 wards. But the general advantages of the chronological 

 method, especially when the reading of such a book as this 

 is rendered really useful by an accompanying actual exami- 

 nation of incubated eggs, are so great that they far outweigh 

 the evil of any such slight irregularities. After tracing out 

 the history of the several organs, no farther than is necessary 

 to give a clear idea of the general course of events in each 

 case, we propose to treat the changes and incidents of the 

 latter days of incubation with great brevity, not attempting 

 any special account of avian development, except in the case 

 of the skull. And even this will be treated summarily. The 

 First Part will therefore really be an introduction to the 

 general facts of vertebrate embryology, the chick being taken 

 as an example. 



In the Second Part we purpose to consider the embryonic 

 histories of other vertebrates, in so far as these differ from that 

 of the bird ; and then to treat of the development of special 

 organs in a more complete manner. 



The Third Part will be devoted to an exposition of the 

 main facts of invertebrate embryology, and to the discussion 

 of general morphological considerations. 



