FROM TWELVE TO THIRTY-SIX SOMITES 199 
dorsal aorta; these are known as the second, third, and fourth 
aortic arches; the third arch is the largest. 
The original mandibular aortic arches unite with the anterior ends 
of the dorsal aorte, forming a loop on each side at the base of the fore- 
brain (Fig. 93), and they have, therefore, a different relation from the 
other aortic arches; it seems probable also that they have a different 
morphological value. The ventral limb of this loop disappears in its 
pre-oral part after this stage and a new vessel is formed entirely within 
the mandibular arch, bearing the same relation to the visceral arch as 
the other aortic arches. At the stage of 35 somites it is a complete arch, in 
some embryos at least (Fig. 117), though of very small caliber and very 
transitory, possibly sporadic, in its occurrence. It is possible that this 
is the true mandibular arch, and the pre-oral portion of the original 
mandibular arch should have another interpretation. Kastschenko 
suggests that it may have been related to lost pre-mandibular gill- 
clefts. 
The roots of the dorsal aorta above the pharynx receive the 
aortic arches and are continued forward as the internal carotid 
arteries, branching in the fore part of the head. Posteriorly the 
right and left aortic roots unite just behind the fourth visceral 
pouch to form the dorsal aorta, and this continues as an undi- 
vided vessel to about the level of the twenty-second somite, 
where it divides into right and left dorsal aorte, and at the 
same time sends out a large omphalomesenteric artery into the 
yolk-sac on each side, and these branch as shown in Figure 115 into 
the capillary network of the yolk-sac. The dorsal aorte, now 
much diminished in size, continue back into the tail where they 
are known as the caudal arteries. The dorsal aorta also sends off 
a pair of segmental arteries into each intersomitic septum, and a 
pair of small allantoic arteries into the primordium of the allantois. 
The veins enter the heart through three main trunks: (1) the 
ductus venosus, (2 and 3) the paired ducts of Cuvier. These 
are made up as follows: (1) the ductus venosus is formed at the 
level of the posterior liver diverticulum by the right and left 
omphalomesenteric veins, which arise in the yolk-sac by union 
of the capillaries of the vascular area; the right vitelline vein 
also receives two veins coming directly from the anterior and 
posterior ends respectively of the sinus terminalis, the anterior 
of these is frequently partly double owing to its mode of origin. 
(See beyond, Chap. VII.) The vascular area in the yolk-saec thus 
