AORTA AND AORTIC ARCHES IN RABBITS 123 



floor of the pharynx and the pericardial cavity, and ultimately 

 reaches the level of the pulmonary anlagen. 



THE AORTIC ARCHES 



The first aortic arch has already been described as a persistent 

 portion of the original angioblast net, folded around the anterior 

 end of the pharynx. The cords of this net become hollow, as is 

 shown by Evans in the drawing of the injected vessels of a duck 

 embryo (Keibel-Mall, vol. 2, fig. 398). Later, as is seen in figs. 

 7 and 8, the net becomes reduced to two vessels on each side, and 

 in the rabbit frequently breaks up into capillaries before being- 

 reduced to a single trunk. 



The second aortic arch (figs. 7 and 8) arises as a lateral exten- 

 sion from the plexus of the ventral aorta, frequently double on 

 one or both sides, consisting at first of solid cords and hollow 

 spaces, and met by much shorter outgrowths from the dorsal 

 aorta. The potentially double character of this arch, even 

 after it has attained a considerable development, is shown in fig. 

 8, x and y. While the second arch is becoming established the 

 plexus of the ventral aorta is extending still further caudad, and 

 again giving off lateral branches, which run between the entoder- 

 mal pouches to the dorsal side of the pharynx, and again are met 

 by shorter growths from the dorsal aorta. Thus the third and 

 fourth arches are formed and, as a glance at fig. 8 will show, they 

 also, from their plexiform origin, are potentially multiple on each 

 side. 



With the growth of the pharynx the conus arteriosus, which 

 joined the ventral aortae originally cephalad to the second arch, 

 has moved caudad, and later opens almost directly into the third 

 and fourth arches. This condition is represented in fig. 9. Here 

 still we see the plexus of the ventral aorta extending caudad from 

 the ventral part of the fourth arch, as it did earlier from the sec- 

 ond arch. Its plexus formation is easily recognized, as it lies 

 in the thin sheet of mesoderm between the pharynx and the peri- 

 cardial cavity, but it no longer crosses the median line on account 

 of the presence of the keel-like growth of the pulmonary anlage 

 from the ventral wall of the pharynx. From the dorsal part of 



