418 Harriet Lehmann, 



The roots of the pulmonary arteries arise from the posterior 

 side of either pulmonary arch, slightly ventral to its middle, and 

 after a short distance, bend toward each other. Especially along 

 the middle part of their course, small spurs extend from these 

 arteries, toward each other. ^) 



The subclavians arise from the aortic trunks, just below the 

 union of the aortic roots. The left fourth arch and the dorsal aorta 

 below it, as well as the left sixth arch, are slightly larger than 

 the corresponding vessels of the right side. The dorsal aorta between 

 the third and fourth arches has diminished noticeably in size, so that 

 it is here less than one-half its diameter posterior to its connection 

 with the fourth arch. 



As in earlier stages, there is also to be seen in this embryo, 

 the persistence of small dorsal rudiments of the first and second 

 arches. These are slightly farther forward, as the embryos become 

 older, but leave the dorsal aortic roots in a similar manner in each case. 



The forward extension of the truncus arteriosus which for the 

 first time is absent in this stage, appears to be peculiar to the pig 

 embryos. No corresponding structure was found in the rabbit, and 

 Tandler describes none either for the human embryo or the rat. 



A change in the relation of the aortic arches to each other in 

 this stage is due mainly to a lengthening of the truncus arteriosus 

 between the fourth and sixth arches. The rudimentary structures 

 betw^een the fourth and sixth arches may for several reasons be 

 considered parts of a forming fifth arch. This arch is completely 

 formed in the next stage, and its posterior portion is very similar 

 to the vessel shown at V in Fig. 22, Plate 24, running close to the 

 dorsal aorta between the fourth and sixth arches. The ventral spur 

 from the systemic arch in this embryo corresponds closely to that 

 in the preceding stage, and to the ventral end of the complete fifth 

 arch in the succeeding embryo. The two entodermal pouches present 

 between the systemic and pulmonary arches are significant. 



The formation of the subclavians agrees with Hochstetter's 

 description for mammals, in the pig as well as in the rabbit. 



Embryo of the twenty- first daj^ The embryo under 

 consideration belongs to the twentj'-first day. Its external anatomy 

 is shown in Fig. 23, Plate 22. Modifications in the contour of the 

 first two branchial pouches are noticeable, and the third pouch is 



1) cf. Bremer, 1902. 



