THE MAIN VESSELS OF THE AREA VASCULOSA. 97 



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 ccelora, and 

 in the splanchnic mesoderm the differentiation of the primitive blood-vessels. These 

 primitive vessels occupy the sharply denned territory, the edge of which is marked 

 by the sinus terminalis (Fig. 131, V.t}. The first differentiation 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 (Fig. 14). Further 

 progress results in the gradual differentiation of the embryo, in the sharp demarca- 

 tion of the area pellucida, which becomes pear-shaped, and in the appearance of 

 the blood-vessels and the resulting differentiation of the area vasculosa or opaca. 

 Figure 131, on page 182, 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.s, 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 em- 

 bryo one easily recognizes the somewhat pear-shaped area pellucida, A.p, and the 

 darker area opaca, by which it is enclosed. The area vasculosa stands out 

 conspicuously and is bounded by the already distinguishable sinus terminalis V.t. 

 Around and underneath is the translucent pro-amnion, pro.am, from which the 

 mesoderm is altogether absent, and which, therefore, cannot contain any blood- 

 vessels. Nor are there at this state any vessels in front of the pro-amnion. The 

 general topographical arrangement is the same in mammals (compare page 179 

 and Fig. 130). 



The Main Vessels of the Area Vasculosa. 



Soon after the capillary network of the areas opaca and pellucida has penetrated 

 the embryo, certain lines of the network begin to widen, and soon distinctly assume 

 the size and functions of main trunks; some of these unite with the posterior venous 

 end of the heart (Fig. 59, Ve.ht), which has meanwhile been formed in the em- 

 bryo, 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 primi- 

 tive circulation starts up. The arrangement of the vessels is not the same in birds 

 and mammals. The disposition in birds is indicated by the diagram shown in 

 figure 53, in which, it should be remembered, the embryo and the capillary net- 

 work are drawn many times too large in proportion to the area vasculosa. The 

 area is bounded by a broad circular vessel, the sinus terminalis, S.T, which con- 

 stitutes a portion of the venous system in birds, for in front of the head of the 

 embryo the sinus leaves a gap, and is reflected back along the sides of the body of 

 the embryo to make two large veins, which, after uniting with the other venous 

 channels coming from various parts of the area vasculosa on each side, enter the 



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