AORTA AND AORTIC ARCHES IN RABBITS 127 



but these plexuses of the pulmonary arteries across the median 

 line are of later occurrence, after much mesoderm has grown in 

 between the trachea and the pericardial cavity. It is unfor- 

 tunate that the pig and the guinea-pig were chosen as illustrations, 

 for, as I have shown in previous papers ('02, '09), in these two 

 species only, so far as I am aware, is such a secondary net found. 

 After the formation of this net one of the connections with the 

 pulmonary arches is lost, so that in these two embryos the trunk 

 of the pulmonary arteries seems very long before the right and 

 left branches are given off. In those two papers I spoke of the 

 pulmonary arteries as branches of the pulmonary arches, a 

 statement which I am now very glad to correct. 



SUMMARY 



In the rabbit, the dorsal aorta, the first aortic arch, the conus 

 arteriosus and the lateral heart are all parts of an original net- 

 work of angioblast cords derived from the extraembryonic plexus 

 of blood-vessels. Those portions of this network which are me- 

 chanically favored in their position persist ; the other portions dis- 

 appear. The favored portions lie (1) under the coelom, (2) under 

 the nephrotome, or (3) under the raised edge of the medullary 

 groove. The connection between first arch and lateral heart 

 is permitted by the lateral expansion of the medullary groove 

 which extends over the coelom. 



This net of angioblast cords is folded in the formation of the 

 pharynx, so that its lateral edge, anterior to the lateral heart, 

 becomes the ventral aorta. In this folding the lateral heart is 

 retarded, and thus comes to lie on a lower plane, more ventral. 

 The connection between the two planes is by means of the conus 

 arteriosus. 



From the net of the ventral aorta a plexus grows mesially and 

 caudally, still forming the ventral aorta. Lateral growths from 

 this pass around the pharynx, often in plexus form, and make 

 the second, third and fourth aortic arches. The conus arteriosus 

 is moved caudad to a position opposite the fourth arch. 



A further extension of the plexus of the ventral aorta, situated 

 between the floor of the pharynx and the dorsal wall of the peri- 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13, NO. 2 



