VITELLINE CIRCULATION 355 



filled with fluid. The sac is ruptured in labor, and the child 

 escapes through the rent. Afterward the decidua vera and 

 placenta are detached, and escape together as the " after birth." 



Development of the Circulation. The development of the 

 circulation may be considered in these stages: (i) Vitelline 

 circulation, (2) placental circulation, (3) adult circulation. The 

 heart is the propelling organ in all these. 



i. Vitelline Circulation. The blood and vessels make their 

 appearance almost as early as the primitive groove. Certain 

 blastodermic cells are transformed into both red and white 

 corpuscles. They are larger than the adult's cells and both 

 are nucleated. Blastodermic cells also group to form small 

 tubes, which constitute the area vasculosa. At the same time 

 mesoblastic cells develop two tubes, one along each side of the 

 body, which soon unite to form a single one, representing the 

 heart. It becomes enlarged and twisted upon itself, and pulsa- 

 tions begin in it at a very early date. The heart is in the median 

 line and gives off two arches which unite below to form the 

 abdominal aorta. From the arches pass branches to the area 

 vasculosa, which now form a nearly circular plexus around the 

 embryo. Two of these branches, larger than the others, enter 

 the umbilical vesicle and become the omphalo-mesenteric arteries; 

 there are two corresponding veins. This circulation through the 

 omphalo-mesenteric vessels and the area vasculosa does not 

 continue long in the human being. As soon as the allantois is 

 formed and the placental circulation begins to be set up, the 

 omphalo-mesenteric vessels are obliterated and the place of the 

 first circulation is taken by the second. 



Development of the Heart. The tube just mentioned as rep- 

 resenting the heart has communicating with it two veins at its 

 lower extremity and two arteries at its upper. Soon the tube 

 becomes twisted upon itself so that the upper (arterial) is thrown 

 in front of the lower (venous). The loop is V-shaped and is 

 the outline of the future ventricles. Afterward a constriction 



