415 



in the condition of the aortic arches. The reconstruction, Fig. 8, 

 shows three complete arches, the third, fourth and sixth, and the first 

 and second arches are also nearly complete. The rudiment of the 

 first arch consists of a long, slender vessel passing downward from the 

 aorta through the mandibular pouch extending to the ventral end of 

 the second arch where it ends blindly. 



The rudiments of the second arch are large, and in reconstruction 

 appear to overlap each other, but this appearance is misleading for 



Fig. 8. ReconstTiction of Aortic arches of Pig embryo of 20th day. a.p. pul- 

 monary arterj'. Aortic arches as numbered. ^ about 30. 



they lie at different planes, and in the reconstruction are projected one 

 against the other. 



The third arch s not relatively as large as in the preceding stage, 

 but the fourth has iicreased in size, making these two vessels nearly 

 equal in diameter. Ihe sixth arch is complete, and shows an en- 

 largement at its dortal end, which joins the aorta upon the inner 

 border of the latter >essel. Each pulmonary arch gives rise, about 

 mid-way in its course, to a pulmonary artery. As in the preceding 



