THE MAIN VESSELS OF THE AREA VASCULOSA. 97 



The Germinal Area. 



The germinal area is that portion of the amniote ovum (mammalian blasto- 

 dermic vesicle) in the center of which the embryo is differentiated. It com- 

 prises, therefore, both the embryo proper and the region immediately surround- 

 ing it. In its center we find the anlages of the embryonic structures proper. In 

 its extra-embryonic part we find the three primitive germ-layers. Underneath 

 the entoderm is the cavity of the yolk-sac. In the mesoderm we have occurring 

 the development of the coelom, and in the somatic mesoderm the differentiation of 

 the primitive blood-vessels. These primitive vessels occupy the sharply defined 

 territory, the edge of which is marked by the sinus terminalis. The first differen- 

 tiation in the germinal area which can be clearly recognized by the naked eye is 

 the appearance of the area pellucida, which is due to the thinning of the entoderm 

 over the Central area. Next ensues the differentiation of the primitive streak. 

 Further progress results in the gradual differentiation of the embryo, in the 

 sharp demarcation of the area pellucida, which becomes pear-shaped, and in the 

 appearance of the blood-vessels and the resulting differentiation of the area 

 vasculosa. Figure 167, on page 296, represents the embryonic area of a hen's 

 ovum after about twenty- seven hours' incubation. The embryo is well advanced 

 in development, for, although the primitive streak, pr, still remains in part and 

 the medullary groove is still open behind, the brain is already marked out and 

 the head has become partly free. Alongside the medullary canal lie eight pairs of 

 segments. Around the embryo one easily recognizes the somewhat pear-shaped 

 area pellucida, A. p, and the darker area opaca, A. o,hy which it is enclosed. 

 The area vasculosa stands out conspicuously and is bounded by the already dis- 

 tinguishable sinus terminalis, st. Around and underneath is the translucent pro- 

 amnion, pro. am, from which the mesoderm is altogether absent, and w^hich 

 therefore, cannot contain any blood-vessels. Nor are there at this state any 

 vessels in front of the pro-atnnion. 



The Main Vessels of the Area Vasculosa. 



Soon after the capillary network of the areas opaca and pellucida has pene- 

 trated the embryo, certain lines of the network begin to widen, and soon dis- 

 tinctly assume the size and functions of main trunks ; some of these unite with 

 the posterior venous end of the heart, which has meanwhile been formed in the 

 embryo, and others become connected with the anterior or aortic end; even 

 before this the heart has begun to beat, so that, as soon as all connections are 

 made, the primitive circulation starts up. The arrangement of the vessels is 

 not the same in birds and mammals, although commonly so stated. The dis- 

 position in birds is indicated by the diagram shown in figure 44, in which, it 

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