656 
vessel has no connection with the dorsal carotid (car. d.). At a later 
stage of development, however, an anastomosis is established between 
the dorsal and ventral carotid. 
Fig. 3 shows the further progress of events in the development 
of the carotid arteries in a chick embryo incubated six and one half 
days. This specimen was injected through the ventricle and the left 
side dissected as in previous specimens. 
The aortic arches have undergone some changes. The sixth arch 
(a. 6), from which springs the pulmonary artery (pl. a.) in a plane 
exactly horizontal to the definitive subclavian (scl. a.), is the largest of 
Fig. 3. A. Camera tracing of dissection of the injected arterial system of the left 
side of a chick embryo incubated six and one-half days, after the establishment of the 
anastomosis between the dorsal and ventral carotid in the region of the hyoid visceral 
arch. >< about 16 diameters. B. Shows details of the relation of the segmental arteries 
to the vertebral and primary subclavian arteries. 
the three. The drawing being made from the left side shows the 
beginning of atrophy of the fourth arch. The dorsal connection between 
of the third and fourth arches is now much reduced. 
The third arch (a. 3) on its lower half gives rise to two vessels 
of about the same diameter. ‘The first, the lower of the two, is the 
definitive subclavian (scl. a.) which passes backward to the wing super- 
ficial to the aortic arches, and running in the same plane with the 
pulmonary artery. It is represented in this figure slightly deflected 
in order to expose the latter artery. The second, located just above 
