CHAPTER XVI. 

 DEVELOPMENT OF THE VA*<TLAR SYSTEM. 



THE vascular system presents itself, during different periods of life, 

 under three distinct forms, which follow each other in the progress 

 of development, as different organs are employed in the functions of 

 nutrition. 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 circulation, in which the mate- 

 rials of nourishment are supplied by the placenta, and which lasts 

 through the greater part of foetal life. The third is the adult circu- 

 lation, in which nutrition and the renovation 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 blood-vessels 

 shoot out from its sides and ramify over the neigh- 

 boring parts of the vitelline sac, forming by their Fio;235. 



inosculation the plexus of the area vasculosa. In the 

 e*r of the fish (Fig. 235), 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 to the embryo by an anterior vitel- "^^^^^ 

 line vein, passing upward alone; the front of the esrff, EGG OF FlsH ( Jarra ~ 



^ bacca >' 



+1 i i ! ^ A u i j ' 



and entering the body beneath the head. vitelline circulation. 



In the egg of the fowl (page 636), the area vascu- 

 losa spreads gradually over the vitelline sac from within outward. 

 During this extension some of its vessels change in relative size and 

 importance. The vena terminalis, forming its outer border, becomes 

 less distinct ; and, in addition to the anterior and lateral vitelline veins, 

 in front and on the sides, there is also a "posterior vitelline vein," 

 coining from the hinder part of the area vasculosa and reaching the 

 embryo beneath its caudal extremity. 



In man and mammalia DS, the first formation of the area vasculosa is 

 similar to that in fishes and birds. But owing to the small size and 

 rapid exhaustion of the vitellus as a source of nourishment, this form 

 of the circulation never acquires a high degree of' development, and 

 soon becomes retrograde. It presents, however, certain modifications, 

 connected with the origin of various parts of the permanent vascular 

 system. 



These modifications relate mainly to the vessels distributing the 



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