460 STRUCTURE OF VERTEBRA! A. 



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 venae 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. The azygos vein of Mammals is 

 a persistent remnant of the 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 connection 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 de- 

 veloped 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 some facts 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 

 Chapter II.). 



Respiratory system. — In Balanoglossus, Tunicates, and 

 Amphioxus, the walls of the pharynx bear slits, between 

 which the blood is exposed in superficial blood vessels to 

 the purifying and oxygenating influence of the water. 



