742 THE CIRCULATORY SYSTEM 



The blood from the tail and posterior trunk region of the body thus must 

 pass through the small blood vessels within the kidney substance. Here waste 

 materials and excess water are extracted before the blood is passed on to the 

 heart and aeration systems. The renal portal system is developed exceptionally 

 well in the embryos and adults of fishes and amphibia. It is inadequately de- 

 veloped in the adult reptile, and it is questionable whether or not the poorly 

 developed, renal portal system functions in the adult bird. The adult mammal 

 does not possess this system. However, the embryos of all reptiles, birds, 

 and mammals possess a renal portal system wherein blood is shunted through 

 the kidney substance from the posterior cardinal veins into the subcardinal 

 complex. It is a most transient affair in the mammalian embryo. The devel- 

 opment of the renal portal system in the anuran embryo is shown in figure 

 335C-E. Observe that pronephric and mesonephric renal portal systems are 

 developed. 



5) Precaval Veins. The formation of the precaval veins is shown in figure 

 335B-G. It is to be observed that the common cardinal veins become trans- 

 formed into the anterior or precaval veins, while the anterior cardinals persist 

 as the internal jugular veins. 



c. Changes in the Primitive Converging Veins of the Heart in the Chick 



1) Transformation of the Vitelline and Allantoic Veins: a) Vitelline 

 Veins. The vitelline veins in the developing chick first make their appearance 

 as two delicate capillaries, one on either side of the inner wall of the anterior 

 intestinal portal in blastoderms of 26-28 hours of incubation. At this time 

 there are about four pairs of somites present. These minute blood vessels are 

 intimately associated with the entoderm of the anterior intestinal portal, and 

 eventually come to lie side by side immediately below the foregut as the anterior 

 intestinal portal recedes caudally. At about 27-29 hrs. of incubation, or when 

 the embryo has about five to six pairs of somites, the two splanchnic layers 

 of the hypomeric mesoderm, in the area where the heart is to form, begins 

 to cup around and enclose the two vitelline capillaries (fig. 332L). A little 

 later, at about 29-33 hrs. of incubation, these two splanchnic mesodermal 

 layers begin to fuse above and below the vitelline capillaries (fig. 332M). 

 At 33-38 hrs. of incubation, or when nine to ten pairs of somites are present, 

 a simple, tubular heart is present which contains the rudiment of the endo- 

 cardium within in the form of the two fused or fusing vitelline capillaries. 

 This endocardial rudiment is enclosed by the hollow, tube-like epimyocardial 

 rudiment derived from the fused layers of splanchnic mesoderm (fig. 336A). 



At about 33-38 hrs. of incubation (fig. 336A), the primitive circulatory 

 system consists of the following; 



( 1 ) Two vitelline veins which converge to enter the forming heart just 

 anterior to the intestinal portal; 



(2) the primitive tubular heart; 



