PISCES 



169 



aortae shortly fuse to form the single aorta, and the vitelline 

 artery is formed later to lead the blood back to the blastoderm 

 from a point above the pronephros on the right side. The 

 blastoderm thus marks the vascular area of the yolk, and as it 

 extends so do the vessels. The artery extends forwards in 

 front of the embryo and is spread into small arteries and into 

 capillaries. The blood is reassembled into veins which finally 

 open into a vein which occupies the margin of the vascular 

 area. The margin, however, is gradually fused behind the 

 embryo, and consequently the two marginal vessels fuse to 



Branch, ves. 



Ao. Proneph. duct 

 Proneph. Notoch. Dors, fin 



Sp. cord 



Postanal 

 gut 



Neurent. 

 canal 

 \ Caudal fin 



Vit. art. Mouth Vent. ao. Heart Enteron Subint. vein Cloaca 

 FIG. 86. Raia. Diagram of the embryonic circulation. 



it. vein 



form a single vitelline vein which lies along the primitive 

 streak and leads the blood into the heart. As it passes into 

 the body it comes into relationship with two veins which bring 

 the blood back from the posterior end of the embryo, the 

 subintestinal veins, and these, like the aortae above, fuse to 

 form a single vessel. The other veins described below are 

 gradually formed as the embryo increases in size. 



The heart lies in the pericardial cavity that is to say, just 

 in front of the pectoral girdle. The venous blood is carried 

 into it by the sinus venosus, which runs across the hinder wall 

 of the cavity between the two precaval sinuses. The sinus 

 venosus opens into the auricle by a two-valved aperture. The 

 large, thin-walled auricle transmits the blood to the ventricle, 

 and the opening is guarded by two valves. The thick-walled 



