CHAPTEE XVII. 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



THERE are three distinct forms assumed by the circulatory system 

 during different periods of life. These different forms of the circulation 

 are connected with the manner in which nutrition and the renovation of 

 the blood are accomplished at different epochs ; and they follow each 

 other in the progress of development, as different organs are employed 

 in turn to accomplish the above functions. The first form is that of the 

 vitelline circulation, which exists at a period when the vitellus is the 

 source of nutrition for the embryo. The second is the placental circula- 

 tion, which lasts, in man and the mammalians, through the greater part 

 of foetal life, and is characterized by the existence of the placenta ; the 

 third is the complete or adult circulation, in which the renovation and 

 nutrition of the blood are provided for by the lungs and the intestinal 

 canal. 



Vitelline Circulation. When the body of the embryo has begun to 

 be formed in the centre of the blastoderm, a number of bloodvessels 

 shoot out from its sides and ramify over the neigh- 

 boring parts of the vitelline sac, forming by their Fig. 298. 

 inosculation an abundant vascular plexus. The area 

 occupied by this plexus around the foetus is the 

 " area vasculosa." In the egg of the fish (Fig. 298), 

 the area vasculosa occupies the whole surface of the 

 vitellus, outside the body of the embryo. A number 

 of arteries pass out from each side, supplying the 

 vascular network; and the blood is returned from EGG OP FISH 

 it to the embryo by a principal vein which is seen (Jarrabacca), show- 



, , , , ing the vitelline cir- 



passmg upward along the front of the egg, and enter- cuiation. 

 ing the body beneath the head. 



In the egg of the fowl, the area vasculosa spreads gradually over the 

 vitelline sac from within outward. It is at first limited on its external 

 border by a terminal vein or sinus, which collects the greater part of 

 the blood from the vascular plexus on each side, and returns it to the 

 interior of the embryo by a double or single trunk, entering, as in the 

 fish, beneath the head. Another vein, of smaller size, enters the body 

 of the embryo near its posterior extremity; and a number of others, 

 still smaller, along the sides. All these vessels gradually change in 

 relative importance, as the development of the embryo proceeds. 

 Especially the terminal sinus becomes less distinct as the area vascu- 

 losa extends farther over the vitelline sac, and the anterior and pos- 



(791) 



