THE EARLY DEVELOPMENT OF THE CHICK 273 



develop here. Thus the cellular or corpuscular elements of 

 the early vascular system arise only in the area opaca, and 

 most extensively in its posterior region. Following Riickert, 

 we may say that the vessels of the pellucid area are formed by 

 the rearrangement of small groups of cells in the splanchnic 

 mesoderm of the area. These first form short sections of tubu- 

 lar vessels which very soon connect with the more peripheral 

 vessels of the opaque area. In this way a continuous vascular 

 network develops centripetally, finally reaching the embryo 

 about the time six pairs of somites are formed. 



Soon after this, vessels appear in the embryo itself. The 

 first of these are the paired dorsal aortce of the body region 

 (Fig. 105). These are to be regarded as enlarged and straight- 

 ened inner (axial) margins of the vascular network of the pellu- 

 cid area. Posteriorly they diverge widely, passing as the 

 vitelline arteries, into the general vascular net (Fig. 107). 

 Anteriorly they become prolonged forward toward the head 

 region, where they connect with a pair of vessels differentiated 

 within the mesenchyme of the head. 



The heart is clearly a specialized portion of the system of 

 vessels. Like the dorsal aortae, the rudiment of the heart is 

 paired. When the vessels grow into the head region, which is 

 now elevated above the level of the general splanchopleure, 

 they are related to the ventral, rather than to the dorsal region 

 (Fig. 105). The amnio-cardiac vesicles (see above) become 

 vascularized in the same manner as the rest of the pellucid 

 area, and a pair of ventral aortce is formed beneath the fore-gut 

 (Fig. 103). Posteriorly to the opening of the fore-gut (anterior 

 intestinal portal) these vessels diverge and pass into the vascu- 

 lar net as the rudiments of the vitelline veins. Reference to 

 the transverse section illustrated in Fig. 105, A, shows that 

 beneath the fore-gut the mesoderm has a considerable vertical 

 extent. This is commonly regarded as splanchnic mesoderm, 

 although it is a region where somatic and splanchnic layers 

 become continuous. The paired rudiments of the heart pass 

 along the inner or axial surface of this vertically extended 

 mesoderm, and thus come into close relation, shortly fusing 



