291 
sections. While there is danger that very fine vessels may be uninten- 
tionally removed during the process of dissection, that can be deter- 
mined by an examination of sections of a control-specimen of the same 
age, and an actual dissection will give more reliable pictures than the 
most carefully made reconstruction. 
Four complete aortic arches the third, fourth, fifth and sixth are 
present on both sides of the embryo. The sixth arch is curved 
caudad, and the pulmonary artery is given off its posterior margin 
about midway between its extremities. The fifth arch is an offshoot 
of the sixth. It is a slender vessel arising from the lower fourth of 
the sixth arch about on a level with the bases of the third and fourth 
arches, and terminates dorsally in the sixth arch not far from the 
point where the latter joins the aortic root. The fifth arch is slightly 
curved cephalad, but in another 
a injected specimen its course is 
ao u straight so that it has the ap- 
re pearance of a cord stretched 
across the bow of the sixth 
arch. The ready passage ofthe 
injecting fluid into it shows, 
however, that it isa true vessel 
and not a solid cord. 
Fig. 1. 
Fig. 1. Camera sketch of a dissection of the injected aortic arches of the left 
side of a chick embryo of four and one-half days incubation. X about 8 diameters. 
The fifth arch is a division of the sixth. 3, 4, 5, 6, in all figures, third, fourth fifth 
and sixth aortic arches respectively. 
Fig. 2. Camera sketch of the same aortic arches. about 35 diameters. 
Fig. 2 shows a camera sketch of the aortic arches of the same 
embryo under a higher degree of magnification. No distinct entodermal 
pouch was observed in the chick between the fifth and sixth arches. 
The wall of the pharynx is slightly curved outward in this region, 
and in one set of sections, the pharyngeal wall shows a small notch, 
at a point between the arches, and a cluster of closely united epi- 
19* 
