Embryonic history of the aortic arches in mammals. 427 



backward in this course an actual connection exists between the 

 two vessels. In Fig-. 28, Plate 25, while a right sixth arch is 

 present, no connection is to be observed from that vessel to the 

 single pulmonary artery, which now arises from the right side of 

 the left sixth arch, just below its union with the sixth arch of the 

 right side. These changes as already indicated agree with the 

 descriptions of Bremer (1902). 



The right sixth arch in the later stages considered becomes 

 smaller than the left, and in Fig 28, Plate 25, is apparently in its 

 final stage as a complete vessel. Its manner of disappearance is 

 similar to that in the portion of the aortic root (connecting the third 

 and fourth arches. The ductus arteriosus is a large vessel in the 

 oldest stage considered, and appears continuous with the pulmonic 

 trunk entering the heart. 



In the earliest stages observed in the rabbit the most anterior 

 point of union of the aortic arches of the two sides lies at the 

 ventral ends of the first pair of arches. In a stage showing complete 

 second and third arches, the remnants of the first pair join either 

 second arch slightly above their union. The third arches join, 

 separately, the large ventral trunk- continued after tlie union of the 

 second arches. In a rabbit of eleven days the second and third 

 pairs of arches appear to unite simultaneously to form a common 

 ventral trunk, which the remaining arches join slightly farther 

 backward, the sixth pair being connected with the ventral ends of 

 the fourth rather than the truncus itself. At eleven and one-half 

 days, the remnants of the second arches join the third arches above 

 the union of the latter vessels. 



In the pig series a forward prolongation of the truncus arteriosus 

 forms the place of attachment for remnants of the second arch. This 

 disappears in the latter part of the twentieth day, and the two 

 third arches unite below the roots of the internal carotids. In the 

 twenty-second day (probably) when short common carotids are present, 

 the most anterior union of the aortic arches is at the ventral ends 

 of the fourth pair, while in the stage shown in Fig. 28, Plate 25, 

 the place of union of the fourth arches comes to lie below the roots 

 of the common carotids and nearer the heart. This might be brought 

 about by an actual splitting in the anterior part of the truncus 

 arteriosus, but these changes are so closely associated with other 

 changes in the relation of the vessels to each other, and to the 



