DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



835 



adopted the parablastic cells are to be distinguished from the cells of the 

 mesoblast, which latter are called archiblastic cells. According to Rind- 

 fleisch the so-called parablastic cells are derived from the area opaca. 



The First, or Vitelline Circulation. In the development of oviparous 

 animals, the first, or vitelline circulation is very important ; for by these ves- 

 sels the contents of the nutritive yelk are taken up and carried to the em- 



FIQ. 313. Area vasculosa (Bisehoff). 

 o, o, 6, sinus terminalis ; c, omphalo-mesenteric vein ; d, heart ; e, f, /, posterior vertebral arteries. 



bryon, constituting the only source of material for its nutrition and growth. 

 In mammals, however, nutritive matter is absorbed almostly exclusive from the 

 mother, by simple endosmosis, before the placental circulation is established, 

 and by the placental vessels, at a later period. The vitelline circulation is 

 therefore not important, and the vessels dissappear with the atrophy of the 

 umbilical vesicle. 



The area vasculosa in mammals consists of vessels coming from the body 

 of the embryon, forming a nearly circular plexus in the substance of the vi- 

 tellus, around the embryon. The vessels of this plexus open into a sinus at 

 the border of the area, called the sinus terminalis. 



In examining the ovum when the area vasculosa is first formed, the em- 

 bryon is seen lying in the direction of the diameter of the nearly circular 

 plexus of blood-vessels. The plexus surrounds the embryon, except at the 

 cephalic extremity, where the terminal sinuses of the two sides curve down- 

 ward toward the head, to empty into the omphalo-mesenteric veins. As the 

 umbilical vesicle is separated from the body of the embryon, it carries the 

 plexus of vessels of the area vasculosa with it, the vessels of communication 



