CHAP. XLII.] OF HEART AND AORTIC ARCHES. 603 



About the sixth day, a considerable alteration takes place in the 

 circulation. By this time, the allantois forms a vesicle of consi- 

 derable size, upon the surface of which numerous vessels are spread 

 out. These are derived from two branches resulting from the 

 division of the aorta after it has given off the mesenteric artery. 

 The allantois rapidly increases in size ; and as the albumen dimi- 

 nishes in quantity, it becomes applied to the membrane of the egg- 

 shell. Through the latter, and through the pores of the shell 

 itself, the air passes to aerate the blood circulating in the vessels 

 of the allantois, which may therefore be looked upon as the great 

 respiratory surface of the chick previous to the formation of 

 lungs. 



About the sixth or seventh day, the heart acquires its character- 

 istic form ; its cavities have approximated more closely, and become 

 conjoined ; the division between the auricles and ventricles can be 

 seen distinctly. The bulb of the aorta appears to arise from both 

 ventricles, immediately over the septum ; and its division into two 

 canals is complete on the seventh day. The pericardium is formed. 

 Only two vascular arches arise on the left side of the aorta, but on 



Fig. 284. 



I, 2, 3. Heart of chick at the 45th, 65th, and 85th hours of incubation. After Dr. Allen 

 Thomson. 4. Heart of a human embryo about the fifth week. After Von Baer. a. Venous 

 trunks, b. Auricle, c. Ventricle, d. Bulbus arteriostis. e. Two aortic arches, which unite 

 posteriorly to form the aorta, g. Auriculo- ventricular opening, h. Septum arising from the 

 lowest part of the cavity of the ventricle. *. Inferior vena cava. 



the right there are three. The latter, and the two anterior arches, 

 are the chief divisions of the aorta, and receive the blood transmit- 

 ted from the left ventricle. On the seventh day, the two posterior 

 arches receive blood only from the right ventricle, and become the 

 pulmonary arteries. At present, however, all the arches terminate 

 in the descending aorta. 



At this period, the course of the blood is as follows : From the 

 system of the embryo it is carried by the arteries vitellince, or 

 omphalo-mesenterica, to the net-work of vessels of the vascular area, 

 whence it passes to the sinus terminalis, which bounds the latter, 

 and which, even on the fourth day, is found to be full of blood. 

 The blood is returned to the heart by two anterior and two pos- 

 terior venous trunks, arranged in pairs on each side of the median 



R R 2 



