144 On the Origin of the Pulmonary Arteries in Mammals 



former course of the right pulmonary artery, but soon even this dis- 

 appears. 



Along with this change, another, common to all mammals, has taken 

 place, namely, the obliteration of the right pulmonary arch; but this 

 is not the cause of the obliteration of the right pulmonary artery, since 

 the lumen of the latter is the first to close. Fig. 8. Still another change 

 is seen, as in the rabbit, in the lengthening of the truncus pulmonalis 

 at the expense of the two pulmonary arches, and the consequent appar- 

 ent movement of the left pulmonary artery toward the right pulmonary 

 arch. In the pig, considerable variation seems to occur in regard to 

 the stage of growth at which this last mentioned change takes place, as 

 may be seen by comparing Figs. 6 and 7, where the distance between 

 the points of origin of the pulmonary arteries is about the same in 

 two pigs of 11.0 and 12.0 mm., respectively, and Fig. 8, where the 

 distance is much greater, although the length of the embryo is again 

 12.0 mm. It will be seen that of the two 12.0 mm. pigs, one still 

 has, and one has already lost, the connection of the right pulmonary 

 artery. 



Whether all ungulates, or only pigs, have this odd method of arriving 

 at the adult relations of the pulmonary arteries, I do not as yet know; 

 certainly there is nothing like it in the rabbit, the cat, the dog, or in 

 the human embryos within my reach. 



