THE EMBRYOLOGY OF THE OPOSSUM 



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aortic arches in the rabbit embryos, in which he points out 

 that the pulmonary arteries are extensions of the ventral 

 aortae not outgrowths of the sixth aortic arch. In fact, 

 they grow past the level of the sixth aortic arch before that 

 arch is formed. This is quite definitely the case in the 

 opossum; Federow (see Bremer, '12 b) has found it in the 

 guinea pig; Buell ('22) found it in the chick; and Congdon 

 ( '22) found it in man. What is, therefore, usually interpreted 

 as the ventral half of the sixth arch is really part of the 

 ventral aorta; and the arch, which is entirely dorsal to the 

 point of origin of the pulmonary artery, is also entirely lost 

 in later development, no part of it ever being incorporated 



Fig. 43 Lateral reconstruction of aortic arches and pulmonary artery in 

 stage 30, 17132. 



into the pulmonary artery. Thus far, the course of events 

 in the origin of the opossum's pulmonary artery coincides 

 with what Bremer found in the rabbit ; but in stage 30 I have 

 found in three specimens a condition not heretofore described, 

 and possibly of considerable significance. 



Figure 43 shows a reconstruction of the aortic arches and 

 pulmonary artery on the left side in specimen 17132. The 

 most interesting feature of this specimen is the presence of 

 a plexiform and not quite complete seventh aortic arch. 

 Whether this is a normal structure I am not prepared to say 

 with finality. The embryo is normal in every other respect. 

 On the right side in the same specimen the rudiment is less 

 well developed but it is represented by a ventral sprout at the 



