688 



HE PRODUCTION. 



FIG. 23(5 



blood to the external vascular plexus, and returning- it thence to the 

 embryo. As the embryo and the entire egg increase in size, two arte- 

 ries and two veins become larger than the rest, and subsequently do 

 the whole work of conveying the blood to and from the area vasculosa. 

 The arteries emerge from the lateral edges of the embryo, on the 

 right and left sides ; while the veins re-enter at about the same point 

 and nearly parallel with them. These four vessels are termed the 

 omphalo-mesenteric arteries and veins. 



The arrangement of the circulatory apparatus in the interior of the 

 body at this time is as follows : The heart is situated at the median 

 line, immediately beneath the head, and in front of the oesophagus. It 

 receives at its lower extremity the united trunks of the two omphalo- 

 mesenteric veins, and at its upper extremity gives off two vessels which 

 almost immediately divide into two sets of lateral arches, bending back- 

 ward along the sides of the neck, and reuniting into two trunks in front 

 of the vertebral column. These trunks then run from above downward 



on each side the median line. They 

 are called the vertebral arteries, on 

 account of their situation, adjacent 

 to and parallel with the vertebral 

 column. They give off, throughout 

 their course, small lateral branches, 

 which supply the body of the embryo, 

 and also two larger branches the 

 omphalo-mesenteric arteries which 

 pass out, as above described, to the 

 area vasculosa. The two vertebral 

 arteries remain separate in the upper 

 part of the body, but fuse with each 

 other a little below the level of the 

 heart ; so that, beyond this point, 

 there remains but one large artery 

 the aorta running from above 

 downward along the median line, 

 giving off the omphalo-mesenteric 



arteries to the area vasculosa, and supplying smaller branches to the 

 body, the walls of the intestine, and the other organs of the embryo. 



This is the condition which marks the first or vitclline circulation. 

 A change now begins to take place, in which the vitellus is superseded, 

 as an organ of nutrition, by the placenta ; giving rise to the second or 

 placental circulation. 



Placental Circulation. After the umbilical vesicle has been formed 

 by the process already described (page 639), a part of the vitellus 

 remains included in it, while the rest is retained in the abdomen, 

 enclosed in the intestinal canal. As these two organs (umbilical vesicle 

 and intestine) are originally parts of the same vitelline sac, they remain 

 supplied by the same vascular system, namely, the omphalo-mesenteric 



Diagram of the EMBRYO AND ITS VESSELS, 

 showing the circulation of the umbilical vesi- 

 cle ; and also that of the allantois, beginning 

 to be formed. 



