Embryonic history of the aortic arches in mammals. 397 



end of the third arch, he regards as related to the shifting of the individual 

 vessels from each other. 



Bremer (1902) gave the first correct account of the earlier history 

 of the pulmonary artery in mammals, prior to the condition described by 

 Rathke. He finds that in the rabbit the pulmonary arteries arise as 

 buds from the mesial aspect of each pulmonary arch. As the truncus 

 pulmonis twists about the aorta as the result of unequal growth, a traction 

 is brought upon the pulmonary arches crowding them together and at 

 the same time drawing together the pulmonary arteries. By fusion of 

 the two parallel arches the truncus pulmonis is increased in length, "and 

 this fusion may extend until the origins of the pulmonary arteries are 

 very near the bifurcation, or until the left artery springs actually from 

 the bifurcation, .... The left pulmonary arch grows rapidly .... 

 while the right becomes entirely obliterated beyond the point where the 

 pulmonary artery arises, .... from this point to the junction with the 

 left arch, the right arch remains of the same calibre as the pulmonary 

 artery". With a few minor changes, "the left pulmonary arch seems to 

 give rise, at about its mid-point, to two arteries with their origins close 

 together (or there may be a very short common stem)". 



In the pig, the pulmonary arteries, after attaining considerable length, 

 bend toward each other, and send out buds which grow into connections 

 between the two vessels until they are united into one channel in a con- 

 siderable part of their length. Meanwhile, the upper portion of the right 

 pulmonary artery which often shows signs of irregularity, degenerates and 

 is obliterated. The obliteration of the right pulmonary arch goes on at 

 the same time, but this does not cause the disappearance of the right 

 pulmonary artery, "since the lumen of the latter is the first to close". 

 It thus appears that Rathke represented only the result of these changes, 

 while Bremer has demonstrated the actual process of formation of the 

 pulmonary arteries for pig and rabbit. 



HOCHSTETTER (1903) published a second summary of the literature 

 upon the development of the blood vascular system. He again gives the 

 number of aortic arches for Amniotes as six, and presents schemes upon 

 this basis. In this paper HoCHSTETTER also furnishes an account of the 

 origin of the subclavian artery, through the vertebrate series, beginning 

 with Anura. In this group the subclavians arise from the dorsal aorta, 

 but on account of a subsequent splitting in the latter vessel, they spring 

 later from the aortic roots, instead of the common trunk. ' In Saura 

 the origin is similar, but the division in the aortic trunk is not symmetrical, 

 the plane of separation turning to the left, so that the dorsal segmental 

 arteries from which the subclavians spring, are separated to the right, 

 thus making the subclavians, which first arise symmetrically from the 

 dorsal trunk, take their origin later from the right aortic root. In 

 Chelonia and Crocodilia a great difference exists. The subclavians which 

 develop in Saurii are probably formed in turtles and the crocodile, but 

 are later replaced by arteries which arise from the ventral ends of the 

 third pair of arches, and which eventually entirely supply the fore-limbs 

 with blood. The birds have a primary subclavian springing from the 



