508 STRUCTURE OF VERTEBRATA. 
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 amce- 
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 lamellz or respiratory 
function. In the embryo the blood is purified, as will be 
explained afterwards, by aid of the foetal 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 
