EMBRYOLOGY OF THE CHICK 329 



The circulation of the blood is somewhat like the circulation in the 

 fetus of quadrupeds. The primitive tubular heart (Fig. 83, No. D, 

 8) is bent in S-shape; the two ends are connected with blood-vessel?; 

 and later, by the development of the septa, the cavities of the adult 

 heart are denned. The foramen ovale is found in the median 

 septum. This opening brings the right and the left side into commu- 

 nication. The septa between the cavities of the heart are completed 

 by the end of the sixth day. The two vitelHne, or omphalo-mesen- 

 teric, veins carry the blood containing the nutrients from the yolk 

 sac to the liver, where it is mixed with the blood drained from the 

 intestines by the portal vein, and this blood is finally carried by 

 the posterior vena cava into the right auricle of the heart. From 

 here it passes through the foramen ovale to the left auricle; from the 

 left auricle it enters the left ventricle, from which it is forced into 

 the aorta, and then out into the systemic circulation. Practically 

 all, if not all, of the blood of the pulmonary artery is sent into 

 the aorta through a connection with this vessel called the ductus 

 arteriosus. 



Down appears about the thirteenth day of incubation. There are 

 two kinds of down on the chick, one long, which comes first, about 

 two or three days before hatching; a second, or fine, down forms at 

 the roots of the other. As the embryo develops the air cell, at the 

 large end of the egg which is developed between the two shell 

 membranes, or membrance putamince, gradually enlarges. 



dibular arch. 5, The hyoidean arch. 6, Scapular arch. 7, The same. 8, The 

 ventricular portion of the heart. 9, The right transverse vitelline vein. 10, 

 Posterior longitudinal vitelline vein, n, Vertebral vein. 12, Tributaries of 

 same. 14, The auricle. 16, Aortic bulb. 



E, A transverse section of chick embryo, 29 hours incubation. I, Neural 

 canal. 2, Neural crest. 3, Somatopleure. 4, Splanchnopleure. 5, Omphalo- 

 mesenteric vein. 6, Aorta. 7, Notocord. 8, Pleuroperitoneal cavity. 



F. Chick embryo at the ninth day of incubation, i, Allantois. 2, Amnion. 

 3, Air cell at large end of the egg. 4, Egg shell. 5, Outer shell membrane. 6, 

 Inner shell membrane. 7, Yolk sac. 8, Albumin (Bradley). 



