GENERATION AND DEVELOPMENT. 281 



Bulbus Arteriosus. The lulbus arteriosus which is originally a 

 single tube, becomes gradually divided into two by the growth of an in- 

 ternal septum,, which springs from the posterior wall, and extends forward 

 toward the front wall and downward toward the ventricles. This parti- 

 tion takes a somewhat spiral direction, so that the two tubes (aorta and 

 pulmonary artery) which result from its completion, do not run side by 

 side, but are twisted round each other. 



As the septum grows down toward the ventricles, it meets and coalesces 

 with the upwardly growing ventricular septum, and thus from the right 

 and left ventricles, which are now completely separate, arise respectively 

 the pulmonary artery and aorta, which are also quite distinct. The au- 

 riculo- ventricular and semilunar valves are formed by the growth of folds 

 of the endocardium. 



At its first appearance the heart is placed just beneath the head of 

 the foetus, and is very large relatively to the whole body: but with the 

 growth of the neck it becomes further and further removed from the head, 

 and lodged in the cavity of the thorax. 



Up to a certain period the auricular is larger than the ventricular 

 division of the heart; but this relation is gradually reversed as develop- 

 ment proceeds. Moreover, all through foetal life, the walls of the right 

 ventricle are of very much the same thickness as those of the left, which 

 may probably be explained by the fact that in the foetus the right ventri- 

 cle has to propel the blood from the pulmonary artery into the aorta, and 

 thence into the placenta, while in the adult it only drives the blood 

 through the lungs. 



Arteries. The primitive aorta arises from the bulbus arteriosus 

 and divides into two branches which arch backward, one on each side of 

 the foregut, and unite again behind it, and in front of the notochord, into 

 a single vessel. 



This gives off the two omphalo-mesenteric arteries, which distribute 

 branches all over the yolk-sac; this area vasculosa in the chick attaining 

 a large development, and being limited all round by a vessel known as 

 the sinus terminalis. 



The blood is collected by the venous channels, and returned through 

 the omphalo-mesenteric veins to the heart. 



Behind this pair of primitive aortic arches, four more pairs make their 

 appearance successively, so that there are five pairs in all, each one run- 

 ning along one of the visceral arches. 



These five are never all to be seen at once in the embryo of higher 

 animals, for the two anterior pairs gradually disappear, while the pos- 

 terior ones are making their appearance, so that at length only three 

 remain. 



In Fishes, however, they all persist throughout life as the branchial 



