290 
and that the question of the homology of the arch from which the 
pulmonary artery springs in mammals is still unsettled. It may be 
added also that the brief description and lack of illustration in Van 
BEMMELEN’S paper gives a very imperfect knowledge of the anatomical 
relationships of the supposed fifth arch in birds. 
The above summary serves to show the present status of the 
question regarding the existence of six aortic arches in higher verte- 
brates. All observers agree that the pulmonary artery arises from the 
posterior arch, but the crucial point may be stated. Is the posterior 
arch in mammals the representative of the sixth arch in other verte- 
brates, and are the irregular arteries occurring between the fourth and 
the pulmonic vestiges of a fifth arch? 
Before making any comment on that phase of the question the 
actual condition of the fifth aortic arch in birds will be described and 
figured. 
Observations on the fifth and sixth Aortic Arches in 
Chick Embryos. As already stated Van BEMMELEN noticed the 
presence in the chick of an arch hetween the fourth and the pulmonic, 
but he did not figure it nor describe its connections. The latter circum- 
stance leaves us in doubt as to whether in his specimen it had connec- 
tions with the systemic or pulmonic or with neither. His statement 
regarding the fifth arch in his paper of 1886 is brief and as follows: 
»Hinter der vierten Kiemenspalte entstehen beim Hiihnchen wie bei 
Reptilien nicht ein, sondern zwei Aortenbogen, von denen der vordere 
nur sehr geringe Mächtigkeit erreicht und sehr bald wieder verschwindet, 
während der hintere, also der sechste, zur Art. pulmonalis wird.“ 
In the interval between Van BEMmMELEN’s paper and the present, 
so far as I am aware, no picture has been published showing the fifth 
aortic arch in birds, and therefore, a figure and more exact description 
of its condition are very desirable. 
Fig. 1 shows a camera sketch of a dissection of the aortic arches 
of the left side of a chick embryo of about four and one-half days 
incubation. The specimen was injected with several others of the 
age by introducing India ink into the liver with a very fine hypo- 
dermic syringe while the heart was still in action (MALL). The India 
ink being taken up by the circulation fills the blood vessels with the 
black fluid, which, after dissection, shows them very distinctly outlined 
against the white background of the tissues. The overlying tissues 
are too opaque to enable one to see the aortic arches plainly without 
dissection. I am indebted to Mr. G. H. Twining, a graduate student 
in my laboratory for the injected material and for a partial dissection 
of the same. Since his work lies in other directions he has no in- 
tention of using the material in this connection. The dissections were 
completed and drawn by the writer. It may be fairly claimed that 
this method has advantages over that of reconstruction from serial 
