AORTA AND AORTIC ARCHES IN RABBITS 



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on the left side of the embryo, an anastomosing plexus. On the 

 other side of this embryo these sprouts from the dorsal aorta are 

 much simpler, though still double, while a lateral extension of the 

 plexus of the ventral aorta replaces that derived, on the left, from 

 the dorsal sprouts. In other embryos there is great variation in 

 these vessels. There can be no doubt that these vessels from the 

 dorsal aorta are about to join the ventral plexus and form another 

 arch; nor can there be any question that in this case there are two 



Fig. 9 Rabbit, 10| days, 3.2 mm., H.E.C., no. 560. Reconstruction and let- 

 tering similar to those of fig. 8. P.art., extension of ventral aortic network as 

 pulmonary arteries. X 125. 



channels on each side. While not wishing to go deeply into the 

 controversy over the presence or absence of a sixth aortic arch, I 

 may say that it seems to me that the final solution should come 

 from a further study of the entodermal pouches, of the branches 

 of the nerves, and of the cartilages in this region. We have seen 

 that the four first arches are potentially double on one or both 

 sides, and we now find that the last arch is so also, with, as is 



