659 
to the upper and lower jaw respectively. We must not lose sight of 
the fact that the latter two branches were derived originally from the 
ventral carotid (car. v.). One having this particular stage under ob- 
servation might readily conclude that the vessels to the upper and 
lower jaw originate from the dorsal vessel (car. d.). 
The anastomosing branch from the dorsal carotid becomes gradu- 
ally larger, and, little by little, more blood passes from the dorsal 
(car.d.) than from the ventral vessel (car. v.) into the branches 
supplying upper and lower jaws. The ventral vessel (car. v.) soon 
shows a tendency to dwindle until at this stage it has completely 
atrophied in its middle portion, so that no longer blood passes to the 
upper and lower jaw through its channel. The anterior remnant 
(car. v) now a spur from the external carotid (car. ex.) passes back- 
ward on the lateral wall of the pharynx but no longer connects with 
the third arch. The posterior remnant, much smaller than the anterior 
one, arises from the third arch just dorsal to the thyroid body and 
runs forward on the lateral wall of the pharynx for a short distance. 
It gradually atrophies, until on the eight day, as was shown by the 
examinations of a large number of specimens, it completely disappears. 
Fig. 5 is a camera tracing of the aortic arches and the carotid 
arteries of the left side at oes 
of a chick embryo on the a eT ee 
eight and one-half day 
of development. 
Fig. 5. Camera drawing 
of dissection of the injected 
aortic arches and carotid arteries 
of the left side of a chick em- 
bryo on the eight and one-half 
day of development, showing 
the branches from the common 
carotid and external carotid 
arteries. X about 8 diameters. ie 
In this stage the third aortic arch has lost its dorsal connection 
with the others and the left fourth arch has entirely disappeared.‘ The 
basal portion of the third arch, between the heart and the subclavian 
(scl. a.), has become the innominate (in. a.). The remainder of the 
third arch in front of the subclavian together with the former dorsal 
carotid (car. d.) now forms the common carotid stem (e. c.). The 
common carotid (c. ec.) after giving off a stem which divides into the 
cervical, vertebral, and arteria colli collateralis branches, passes for- 
ward and bifurcates into its internal and external carotid divisions. 
42* 
