FROM TWELVE TO THIRTY-SIX SOMITES 205 
the diencephalon. They le internal to the cranial nerves and 
pass just beneath the auditory pits. 
As the brain develops many branches of the anterior cardinal 
veins arise, the most conspicuous of which at seventy-two hours 
are a large branch just behind the auditory sac, one between the 
auditory sac and the trigeminal ganglion, an ophthalmic branch 
extending along the base of the brain to the region of the optic 
stalks and a network of vessels on the lateral surfaces of the 
fore-brain. The other branches of the anterior cardinal vein 
are the three anterior intersomitic veins (Fig. 115); the external 
jugular from the floor of the pharynx enters the duct of Cuvier 
just beyond the union of the anterior and posterior cardinal veins. 
Up to about forty-eight hours the anterior cardinal veins lie 
median to the cranial nerves, but between this time and seventy- 
two hours the facial and glossopharyngeal nerves cut completely 
through the vessel and thus come to lie median to it; the trigem- 
inus and vagus continue to lie lateral to it. 
The posterior cardinal arises as a posterior prolongation from 
the duet of Cuvier and grows backward above the Wolffian duct, 
keeping pace with the differentiation of the intermediate cell- 
mass, as far as the thirty-third somite. It does not enter the 
caudal region of the body. As already described it receives 
twenty-nine intersomitic veins and the veins of the Wolffian 
body. At first its connection with the duct of Cuvier is by 
means of a network of vessels, which gradually gives place to a 
single trunk (cf. Fig. 117). 
The Splanchnic Veins. The ductus venosus is the unpaired 
vein immediately behind the sinus venosus, formed by fusion of 
the two omphalomesenteric veins. It is fully formed at the stage 
of 27 somites. Its relations to the liver have already been de- 
scribed in connection with that organ. Its subsequent changes 
are described in Chapter XII. 
The vitelline veins are united at about the stage of seventy- 
two hours by a loop passing over the intestine immediately 
behind the pancreas. (See Chap. XII.) 
VII. Tue Bopy-caviry AND MESENTERIES 
The origin of the dorsal and ventral mesenteries was con- 
sidered in the section of this chapter dealing with the ali- 
mentary canal. As noted there, the dorsal mesentery extends 
