FORMATION OF THE EMBRYO IN THE FOWI/S EGG. 637 



from without inward, occupying successively different parts of the area 

 pellucida, and finally reaching the body of the embryo, into which they 

 penetrate by its lateral edges. 



The main features of the development of the blood and blood-vessels 

 are as follows : First. Their formation commences in what afterward 

 becomes the area vasculosa ; that is, a part of the blastoderm lying on 

 the surface of the yolk outside the body of the embryo. Secondly. 

 The walls of the newly-formed vessels, when they first become pervious 

 to the blood, consist of a single layer of flattened cells, representing 

 the endothelium of the adult blood-vessels. These endothelial tubes 



FIG. 191. 



AREA VASCULOSA OF THE EMBRYO CHTCK, after three and a half days of incubation. 1, 1. Vitel- 

 line arteries. 2, 2, 2. Vena terruinalis. 3. Anterior vitelline veins, right and left. 4. Right 

 lateral vitelline vein. From preparations by Prof. William Hailes. 



extend by budding and proliferation through the area pellucida into the 

 body of the embryo, where they become continuous with the endothe- 

 lial lining of the embryonic blood-vessels and heart ; and the muscular 

 and fibrous coats, of the heart and blood-vessels everywhere, are after- 

 ward produced by a development of new tissue around the primitive 

 vascular channels. Thirdly. The first movement of red blood is from 

 the area vasculosa toward the embryo. For the heart, when its pulsa- 

 tions begin, during the second day of incubation, contains only a color- 

 less fluid ; the red globules being then accumulated in the place of their 

 formation, that is, the meshes of the area vasculosa. But by the move- 

 ment of the colorless plasma, under the impulsive force of the heart's 

 action, the red globules become detached from their resting-places and 

 gradually mingled with the circulating current. 



When the system of the area vasculosa is fully established, on the 

 third day of incubation, the body of the embryo is surrounded by a 



