420 Harriet Lehmann, 



of similar form and relations, but somewhat smaller, is present upon 

 the right side. Two distinct branchial pouches are discernable 

 between the fourth and sixth arches, upon both sides. 



The left sixth arch is large, and of somewhat greater size than 

 the right, and a separation has taken place in the truncus arteriosus 

 dividing the pulmonary from the aortic channel. The pulmonary 

 arteries are larger than in the preceding stage, and the two vessels 

 are nearer each other at their origins. They approach each other 

 rapidly in their backward course, and at least one connection has 

 formed between the two arteries. This agrees with Bremer's 

 description. My own observations upon the pulmonary artery of the 

 pig were completed before seeing his paper. 



The right root of the dorsal aorta is smaller behind the fourth 

 arch, than the left. The right fourth, fifth and sixth arches are 

 also smaller than those of the opposite side. 



The subclavians in this embryo arise at the point of union of 

 the aortic roots. 



It may be safely stated that at least small dorsal remnants 

 of the first and second arches persist up to this stage. A definite 

 external carotid is now present, and the third arch and anterior 

 prolongation of the dorsal aorta indicate the course of the internal 

 carotid. The decrease in size of the dorsal aorta between the third 

 and fourth arches foreshadows the disappearance of that portion of 

 the aortic root. 



The vessel described as a fifth aortic arch in this embryo arises 

 at about the same stage as in other mammals, as described by 

 Zimmermann and Tandler. The presence of an extra entodermal 

 pouch in this region and the distinct character of the vessel will 

 not admit of its being interpreted as a part of the fourth arch or 

 aortic root, and furnish good evidence toward the acceptance of this 

 artery as a fifth aortic arch. 



The relation of the arches to each other is similar to that in 

 the preceding stage. The separation of the aortic and pulmonary 

 trunks appears at this stage for the first time in their developmental 

 history. 



Embryo of the twenty-second (?) day. This embryo is 

 clearly older than the preceding one, but I am unable to estimate 

 its age with much certainty. It does not correspond closely to the 

 pig embryos of the twenty-second day shown in Keibel's charts, 

 and the ages of his older embryos are not stated. 



