FROM TWELVE TO THIRTY-SIX SOMITES 205 



the diencephalon. They lie 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 duct 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. THE BODY-CAVITY 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 



