VASCULAR SYSTEM. 457 



each side unite to form the short transverse ductus Cuvieri, the 

 two ducts entering the sinus venosus of the heart. In Fishes the 

 superior cardinals persist, the inferior cardinals bring back blood 

 from the kidneys, and also to some extent, by means of their 

 union with the caudal vein, from the posterior region of the body. 

 In some cases this union with the caudal is only indirect, through 

 the medium of the kidney (Elasmobranchs), in this way the renal 

 portal system is constituted. In higher Vertebrates before 

 development is completed, the superior cardinals are replaced by 

 the superior vense cavse (into which the superior cardinals open 

 as external jugulars). The inferior cardinals at first return blood 

 from the Wolffian bodies and the posterior region, later they 

 atrophy, and are replaced by an unpaired inferior vena cava 

 which brings back blood from the kidney (efferent renals), from 

 the liver (hepatics), and from the hind limbs except when there 

 is a renal portal system. In Mammals the azygos vein persists 

 as a remnant of inferior cardinals. 



(c) In Amphibia a vein known as the epigastric (anterior abdominal) 

 carries blood from the hind limbs into the hepatic portal system. 

 This vein also receives blood from the allantoic bladder, a fact 

 which is of great theoretical importance. In all higher Verte- 

 brates in embryonic life, the blood from the allantois passes 

 through the liver, and to a greater or less extent into its capil- 

 laries, on its way to the heart. In Reptiles, the allantoic veins 

 persist throughout life as the epigastric vein or veins. In Birds 

 and Mammals, on the other hand, they atrophy completely at the 

 close of fcetal life. In Birds, however, a vein is developed which 

 connects the veins coming from the posterior region with the 

 allantoic veins, this persists when the remainder of the allantoic 

 veins atrophy, and thus in Birds as in Amphibia there is a connec- 

 tion between the components of the inferior vena cava and the 

 portal system. In Mammals no such connection occurs. 



According to many authorities, the vascular system is developed 

 in the mesoblast from the hollowing out of strands of cells, the 

 outer cells forming the walls of the vessels, the inner forming 

 the constituents of the blood. According to some, however, the 

 endoderm plays an important part in the process. Thus, in 

 Elasmobranch fishes, the aorta and the sinus venosus arise directly 

 from the archenteron, and the cardinal veins arise from the fusion 

 of segmental outgrowths of the aorta. 



Associated with the vascular system is the spleen, which 

 appears to be an area for the multiplication of blood cor- 

 puscles. It is usually believed to be of mesodermic origin, 

 but there are some facts which point to its being endodermic. 



Developed in mesoblastic spaces, and continuous with 

 the body cavity on the one hand, and the blood vessels on 

 the other, is the system of lymphatic spaces and vessels (see 

 Chap. II.). 



