412 



dorsal aorta is slightly larger than the right, especially behind the 

 region of the aortic arches. The subclavian arteries, not included in 

 reconstruction, are present in this embryo, and arise from the dorsal 

 aorta in connection with segmental arteries, at a considerable distance 

 behind the union of the aortic roots. 



Embryo of eleven and one-half days. Fig. 5 shows a 

 reconstruction of the aortic arches on the left side of this embryo. 

 The second arch is represented by dorsal and ventral rudiments. The 



.A' 



Fig. 6. Reconstruction of Aortic arches in Rabbit embryo of 12*/^ days. Aortic 

 arches as numbered, ver. vertebral artery, y^ about 20. 



third arch is still a large vessel, but relatively not of as great size 

 as in the preceding embryo. The fourth arch has increased in size 

 and nearly equals the third in diameter. 



In the space between the fourth and sixth arches are structures 

 which appear to be elements of an incomplete arch. They are present 

 on both sides of the embryo. These elements do not fully agree 

 with Zimmermann's description of a fifth arch for the rabbit. He 

 found a complete arch, with somewhat ditierent relations, between the 

 fourth and sixth archs in an embryo of about the eleventh day of 



