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 

 A.m. s. (A. o.m.), 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. V.i.e.d., 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. r.m., 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 pulmonary trunk, the latter into the 

 aortic trunk. The carotid arteries arise from the base of the 

 aortic arch and convey the blood to the head, and also, by way 

 of the subclavians, to the walls of the thorax and to the wing. 

 The left aortic arch has disappeared, and the right arch is con- 



