410 



part of their course. The second aortic arches are large vessels with 

 well established unbroken walls. The third pair are not symmetrical^ 

 the arch of the left side being considerably larger than that of the 

 right. The vessel running forward from the dorsal end of the third 

 arch is flattened against the brain-wall and the reconstruction gives 

 an exaggerated idea of its calibre. 



Embryo of the eleventh day. The next stage is represen- 

 ted in Fig. 4, by an embryo of the eleventh day of development. No 

 trace remains of the first arch, except a slender vessel, which passes 

 forward into the mandibular pouch from the ventral end of the hyoid 

 vessel. The two sides of the embryo are symmetrical in this respect. 

 The second aortic arch is incomplete, having degenerated in its middle 

 region, and shows only a dorsal and ventral rudiment. The third arch 



Fig. 4. Reconstruction of Aortic arches in Rabbit embiyo of 11th day. Aortic 

 arches as numbered. X about 30. 



is greatly increased in size, and is relatively larger than in any other 

 stage observed. The fourt arch is less than one-half the diameter of 

 the third arch and its walls are not entirely uniform. 



From the ventral end of the fourth arch upon both sides a spur 

 extends dorsally, and receives a slender vessel, which has taken its 

 course along more than half the length of the fourth arch, and a short 

 distance exterior to it. Opposite this ventral spur, the dorsal aorta 

 shows a distinct evagination from its ventral side. These structures 

 should be compared with those present between the fourth and sixth 

 arches in the succeeding stages. A comparatively large bud of the 



