. The development of the chick; an introduction to embryology. Birds -- Embryology. 374 THE DEVELOPMENT OF THE CHICK wall, inasmuch as direct connection with the veins of the allantois is not yet established. The meatus venosus receives the omphalo- mesenteric veins, and the blood of the allantois by way of the subintestinal vein (the latter arrangement of very brief duration). Thus at this time all of the blood is mixed together in the sinus venosus, viz., that re- ceived through the ducts of Cuvier, presumably venous, and that received through the meatus venosus, pre- sumably arterial, owing
. The development of the chick; an introduction to embryology. Birds -- Embryology. 374 THE DEVELOPMENT OF THE CHICK wall, inasmuch as direct connection with the veins of the allantois is not yet established. The meatus venosus receives the omphalo- mesenteric veins, and the blood of the allantois by way of the subintestinal vein (the latter arrangement of very brief duration). Thus at this time all of the blood is mixed together in the sinus venosus, viz., that re- ceived through the ducts of Cuvier, presumably venous, and that received through the meatus venosus, pre- sumably arterial, owing to its circulation in the superficial vascular network of the yolk- sac. Apparently there is no arrangement for separation or discrimination in the re- distribution of the blood. But on the other hand it should be noted that most of the blood comes from the yolk-sac, owing to the slight. A/s. Fig. 215. — Region of the bifurcation of the post-cava in the adult fowl. Ven- tral view. (After Miller) development of the vessels s. (A. ), Omphalomesenteric , ,'^ . ^^i- x- artery. A. i. s., Left internal iliac artery, of the embryo at this time, V. c. i., Vena cava inferior. V. i. c. d., Right common iliac vein. , Right external iliac vein. V. i. i. d., Right inter- nal iliac vein. V. i. 1. s., Left vena in- tervertebralis lumbalis. V. sr. s., Left suprarenal vein. Vv. g., Genital veins. Vv. , Great renal veins. and that the blood of the embryo itself cannot be highly venous owing to the shortness of the circuit and the delicate nature of the embryonic tissues, which, no doubt, permit direct access of oxygen. On the sixth day the embryonic circulation enters on a second phase, owing to the changes in the structure of the heart and arrangement of the vessels described in detail in the preceding part of this chapter. On the eighth day the circulation is as follows: The right and left ventricles are completely separate, and the^ former pumps the blood into the pulm
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