430 GENERATION. 



again changed. The placental circulation is then abolished, 

 and the arrangement of vessels peculiar to it disappears. 

 Now, for the first time, the pulmonary circulation becomes 

 important. All the blood passes through the lungs before 

 it is sent to the general system, the two sides of the heart 

 become completely separated from each other, and the third, 

 the pulmonary, or adult circulation, is established. 



The First, or Vitelline Circulation. In the develop- 

 ment of oviparous animals, the first, or vitelline circulation 

 is very important ; for, by these vessels, the contents of the 

 nutritive yolk are taken up and carried to the embryon, con- 

 stituting the only source of material for its nutrition and 

 growth. In mammals, however, nutritive matter is absorbed 

 almost exclusively from the mother, by simple endosmosis 

 before the placental circulation is established, and by the pla- 

 cental vessels, at a later period. The vitelKne circulation is 

 therefore not important, and the vessels disappear with the 

 atrophy of the umbilical vesicle. 



The area vasculosa, in mammals, consists of vessels com- 

 ing from the body of the embryon, forming a nearly circular 

 plexus in the substance of the vitellus, around the embryon. 

 The vessels of this plexus open into a sinus at the border 

 of the area, called the sinus terminalis. It is probable that 

 these vessels are developed de novo in the intermediate blas- 

 todermic layer, and are not preceded by a distinct mem- 

 brane ; but such a membrane has been described under the 

 name of the vascular blastodermic layer. 1 



If we examine the ovum when the area vasculosa is 

 first formed, we see the embryon 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 

 downward toward the head, to empty into the omphalo- 

 mesenteric veins. As the umbilical vesicle is separated from 



1 BURDACH, Trait^ de physiologic, Paris, 1838, tome iii., p. 502, et seq. 



