DEVELOPMENT OF THE CIRCULATORY APPARATUS. 665 



CHAPTER XVII. 



DEVELOPMENT OF THE CIRCULATORY 

 APPARATUS. 



THERE are three distinct forms or phases of development assumed 

 by the circulatory system during different periods of life. These 

 different forms of the circulation are intimately connected with the 

 manner in which nutrition and respiration, or the renovation of the 

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

 other in the progress of development, as different organs are em- 

 ployed in turn to accomplish the above functions. The first form 

 is that of the vitelline circulation, which exists at a period when the 

 vitellus, or the umbilical vesicle, is the sole source of nutrition for 

 the foetus. The second is the placental circulation, which lasts 

 through the greater part of foetal life, and is characterized by the 

 existence of the placenta; and 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. 



First, or Vitelline Circulation. It has already been shown, in a 

 previous chapter, that when the body of the embryo has begun to 

 be formed in the centre of the blastodermic membrane, a number 

 of bloodvessels shoot out from its sides, and ramify over the 

 remainder of the vitelline sac, forming, by their inosculation, an 

 abundant vascular plexus. The area occupied by this plexus in the 

 blastodermic membrane around the foetus is, as we have seen, the 

 " area vasculosa." In the egg of the fowl (Fig. 252), the plexus is 

 limited, on its external border, by a terminal vein or sinus the 

 " sinus terminalis ;" and the blood of the embryo, after circulating 

 through the capillaries of the plexus, returns by several venous 

 branches, the two largest of which enter the body near its anterior 

 and posterior extremities. The area vasculosa is, accordingly, a 

 vascular appendage to the circulatory apparatus of the embryo, 

 spread out over the surface of the vitellus for the purpose of absorb- 

 ing from it the nutritious material requisite for the growth of the 



