CHAPTER II. 



A BRIEF SUMMARY OF THE WHOLE HISTORY OF INCUBATION. 



1. Step by step the simple two-layered blastoderm de- 

 scribed 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 incubation. 



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. 



2. The blastoderm at starting consists of two layers. 

 Very soon a third layer makes its appearance between the 

 other two. These three 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. 



This triple division corresponds roughly, though not exactly, to the old 

 division into serous, vascular and mucous layers. 



3. The blastoderm which at first, as we have seen, lies 

 like a watch-glass over the segmentation-cavity, its margin 

 resting on the circular germinal wall of white yolk, spreads, 

 as a thin circular sheet, over the yolk, immediately under 

 the vitelline membrane. Increasing uniformly at all points 

 of its circumference, the blastodermic expansion covers more 

 and more of the yolk, and at last, reaching its opposite pole 



