CHAPTER II. 



A BRIEF SUMMARY OF THE WHOLE HISTORY OF 

 INCUBATION, 



Step by step the simple two-layered blastoderm 

 described in the previous chapter is converted into the 

 complex organism of the chick. The details of the 

 many changes through which this end is reached will 

 perhaps be rendered more intelligible if we prefix to the 

 special history of them a brief summary of the general 

 course of events from the beginning to the end of incu- 

 bation. 



In the first place, it is to be borne in mind that the 

 embryo itself is formed in the area pellucida, and in the 

 area pellucida alone. The area opaca in no part enters 

 directly into the body of the chick; the structures to 

 which it gives rise are to be regarded as appendages, 

 which sooner or later disappear. 



Germinal layers. The blastoderm at starting con- 

 sists of two layers. Very soon a third layer makes its 

 appearance between the other two. These three layers, 

 known as the germinal layers, the establishment of which 

 is a fact of fundamental importance in the history of the 

 embryo, are called respectively the upper, middle and 

 lower layers, or epiblast, mesoblast and hypoblast. Of 



