thelial cells in the tissue lateral to the fifth and sixth arches. Besides 
these vestiges of uncertain meaning, there are four well developed 
entodermal pouches: two in front of the third arch, and one each 
between the third and fourth, and the fourth and combined fifth and 
sixth arches. The aortic arches on the opposite (right) side of this 
embryo give a similar picture, the fifth arch is a complete vessel 
having a similar arrangement with reference to the sixth; it is rela- 
tively longer and less curved, but it becomes filled with the injecting 
fluid showing that it has a lumen. 
The same condition has been observed in five specimens all in- 
cubated between four and five days. Three of these were injected 
and dissected and two were sectioned. In the three injected specimens 
the fifth arch shows as a complete vessel, and in all it is an offshoot 
of the sixth. This condition of the fifth arch is very similar to that 
illustrated in the reptiles by VAN BEMMELEN and PETER (cf. Fig. 7). 
Study of serial sections bears out the interpretation put upon 
surface views. In both specimens which were sectioned, the fifth arch 
was present only on one side, in one specimen on the right and in 
the other on the left. Figs. 3, 4 and 5 show sections, enlarged about 
48 diameters, through the region of the aortic arches in an embryo of 
nearly five days of development. This specimon was sectioned in my 
laboratory, and a graphic reconstruction showing the fifth and sixth 
arches was made in 1899, but not used in any publication. In Fig. 3, 
on the left side, is seen the dorsal union between the fifth and sixth 
arches. Fig. 5 represents a section taken at the level of the pulmo- 
nary artery which is a posteriorly directed branch of the sixth arch. 
In front, the dorsal carotid is seen connected with the third arch. 
Fig. 4 shows a section at a lower plane where the fifth and sixth 
arches reach their widest divergence. In this specimen the fifth arch 
is placed laterally to the sixth arch instead of anterior to it as shown 
in Fig. 1, and also the distance between the arches is relatively slight. 
In the other specimens examined the arrangement was as in Fig. 1. 
The fifth arch showed in twenty-one sections, each 10 microns thick, 
and by calculation this would make it about one-third as long as the 
sixth arch in the same specimen. 
The histological structure of the fifth arch is like that of the 
other aortic arches, possessing an intima, circularly arranged muscles 
and an extremely thin covering of connective tissue. In its anatomy 
and histology it agrees in all essential features with that of the other 
arches, and, therefore, we conclude that it is a veritable aortic arch 
which makes its appearance in chick embryos between the fourth and 
