291 ANIMAL PHYSIOLOGY. 



each lii like manner divided into right and left cavities. But 

 the truncus arteriosus originally passes up and splits into 

 two divisions, from which come off five pairs of branchial 

 arches, not, however, all existing at one time, but the fore- 

 most of them disappearing as the hinder arches come into 

 view. These arches, which seem to correspond with the 

 branchial processes, pass partially round the oesophagus, and 

 open into two vessels which descend side by side in front of 

 the vertebral column, the primordial aortce. The two aortse 

 afterwards become fused into one, which continues at first to 

 arise by a right and left root, and permanently does so in 

 the lower classes of vertebrates; but in birds the right root 

 alone remains, and in mammals only the left root, while the 

 other is obliterated. The fourth branchial arch of the left 

 side remains in mammals as the arch of the aorta, and the 

 fifth is that from which the pulmonary artery is developed. 



215. At the period of the closure of the amnion, while the 

 yelk sac is still widely connected with the intestine, the allan- 

 toiSj already mentioned, makes its appearance in front of the 

 embryo, at the lower end of the body (fig. 156). It rapidly 

 expands into a large vesicle covered with blood-vessels, and 

 soon becomes constricted in the middle. The vesicular part 

 at the base remains permanently as the urinary bladder; the 

 constriction is termed the urachus, and emerges at the open- 

 ing which is gradually narrowed to form the navel; and the 

 vesicle beyond continues to expand till it reaches the chorion, 

 then, becomes flattened on the inner surface of that mem- 

 brane, and furnishes blood-vessels which penetrate into all 

 its villi. In mammals, a great development of blood-vessels 

 takes place at one or more parts, varying in different 

 families; and at these parts the vessels of the uterine mucous 

 membrane likewise expand to form, in conjunction with the 

 allantoic structure, the placenta, which consists essentially 

 of aggregated tufts of blood-vessels belonging to the foetus, 

 projecting into sinuses filled with maternal blood, whence 

 they suck up supplies both of nourishment and oxygen. 

 In the human subject, the placenta is circular and limited to 

 an area which, at the full term, is only about 7 or 8 inches 

 in diameter. In birds and reptiles the allaiitois spreads out 

 as in mammals, but retains its membranous and hollo vr 



