126 JOHN LEWIS BREMER 



known, great variation in the points of origin and forms of 

 anastomosis. We know that the fourth pouch is forked at the 

 end, but so is also the third pouch. Branches of the vagus have, 

 in a few instances, been found which seem to indicate the pres- 

 ence of an extra arch; but aberrant nerve branches are not 

 unknown elsewhere. As far as the early development of these 

 vessels is concerned, there is nothing certainly to prove the pres- 

 ence of an interpolated arch. 



THE PULMONARY ARTERY 



It will be noted that the sprouts for this last arch arise chiefly 

 from the dorsal vessels, instead of from the ventral net. I also 

 wish to point out that the net grows beyond the arch, before 

 the arch has become complete. In other words this extension of 

 the ventral aortic net forms well defined pulmonary arteries, one 

 on each side, before the pulmonary arch exists; the pulmonary 

 artery is in no sense a branch of the pulmonary arch, and more- 

 over, in the strictest sense, the arch extends only from the dorsal 

 aorta to the pulmonary artery, the ventral part of the vessel 

 usually called the arch is really the ventral aorta. The persist- 

 ent pulmonary arteries are entirely ventral; they have been 

 joined during embryonic life by branches from the dorsal aorta, 

 but such branches are only temporary. 



Here I must add a few words in regard to some recent state- 

 ments on the development of the pulmonary arteries. Evans 

 ('09, fig. 21) gives a figure of the injected pulmonary arteries of a 

 pig embryo of 12 mm., and in the German edition of the Keibel- 

 Mall Embryology he copies (fig. 396) a figure of Fedorow showing 

 the pulmonary arteries of a guinea-pig embryo of twenty-one 

 days. In both cases the vessels form a narrow plexus in front of 

 the trachea, and Evans 2 concludes that here, as in other growing 

 vessels, the main trunks are preceded by a capillary net. That 

 he is correct in the main, that the pulmonary arteries do arise as 

 the extension of the net of the ventral aorta, we have just seen; 



2 Dr. Evans has very readily and kindly acknowledged his error to me; Fedorow 

 did not make the misinterpretation. 



