356 REPRODUCTION 



rion, covered by the decidua reflexa; these last named (cho- 

 rion and reflexa) are in turn forced everywhere against the 

 decidua vera. The result is that all four become practically 

 one membrane, though the union between amnion and cho- 

 rion is not so close as that between the other layers. These 

 membranes constitute, then, a sac 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 the 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 pulsations 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-me sent eric arteries; these are two 

 corresponding veins. This circulation through the ompha- 

 lo-mesenteric vessels and the area vasculosa does not con- 

 tinue 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 



