130 EDWARD MCCRADY, JR. 



same level as the one on the left. In another specimen of the 

 same litter (17156) there are well-developed dorsal and ven- 

 tral sprouts on both sides. And finally, in one specimen 

 (17135) from another litter there are small ventral sprouts 

 on both sides at the proper level. 



The fact that these rudiments have been found in three 

 embryos of the same stage of development, and taken from 

 two litters, suggests that they may be normal structures of 

 short duration. This interpretation is strengthened by the 

 evidence already presented that the lungs are first formed as 

 a pair of gill pouches. For if so, the assumption that at some 

 time in the phylogeny of the pulmonates there must have 

 been a seventh aortic arch, is a necessary part of the theory. 



I therefore follow Bremer in saying that the pulmonary 

 arteries are certainly not derivatives of the sixth aortic arch, 

 and that portion of what has been called the sixth below the 

 origin of the pulmonary artery is really part of the ventral 

 aorta. But I go one step further and suggest that beyond that 

 point the pulmonary artery may represent still another aortic 

 arch, the seventh or true pulmonary arch, the dorsal com- 

 ponent of which in the embryology of most pulmonates has 

 been lost or reduced to a vestige. 



The liver cords. The originally simple and broad ventral 

 diverticulum from the duodenum is now becoming organized 

 into several anlagen which will have very different fates. 

 The portion which runs most nearly craniad is the hepatic 

 duct (fig. 42, reconstruction), and from it many solid cords 

 of cells are growing out into the mesenchyme of the ventral 

 mesentery. These cords ramify in the direction of the sinus 

 venosus and between the most proximal portions of the 

 omphalomesenteric veins, which latter respond to the presence 

 of the cords in a remarkable way. Numerous twigs grow out 

 from the veins and branch among the cords. In the case of the 

 left vein this process rapidly breaks up the original channel 

 into a complex of anastomosing sinusoids running between 

 the cords. 



