508 STRUCTURE OF VERTEDRATA. 



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 foetal 

 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 con- 

 nection 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. The heart, with 

 the exception of its endothelial lining, is a tubular de- 

 velopment of the splanchnic mesoderm. 



Associated with the vascular system is the spleen, which 

 appears to be an area for the multiplication or destruction 

 of blood corpuscles. 



The lymphatic system, developed in mesoblastic spaces, 

 is a special part of the vascular system. It consists of fine 

 tubes which end blindly in the tissues and drain off fluids, 

 of larger vessels which the tubes combine to form, and 

 which open into veins. The lymph vessels contain amoe- 

 boid cells, and have associated lymphatic glands in which 

 these lymphocytes are produced. 



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. 



In Cyclostomata, Fishes, all young and some adult Am- 

 phibians, there are not only clefts on the walls of the 

 pharynx, but gills associated with these. On the large 

 surface of the feathery or plaited gills, the blood is exposed 

 and purified. 



In Reptiles, Birds, and Mammals, traces of gill-clefts 

 occur in the embryos, but without lamellae or respiratory 

 function. In the embryo the blood is purified, as will be 

 explained afterwards, by aid of the fcetal sac known as the 

 allantois ; and after birth the animals breathe by lungs. 

 All adult Amphibians also have lungs, to which the lung or 

 swim-bladder of Dipnoi is physiologically equivalent. 



The gill-clefts arise as outgrowths of the endodermic gut 

 which meet the ectoderm and open. The ventral paired 



