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 aortae, 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. Posteriori}^ the 

 right and left aortic roots unite just behind the fourth visceral 

 pouch to form the dorsal aorta, and this continues as an uncli-. 

 vided vessel to about the level of the twenty-second somite, 

 where it divides into right and left dorsal aortse, 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 1 15 into 

 the capillary network of the yolk-sac. The dorsal aortae, 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 (umbilical) 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-sac thus 



