106 



OBSTETRIC J L PH 1 'SIOL 00 V, 



two primitive aortae, and shows at its posterior termination the small 

 pelvic vessels and the large umbilical arteries. 



The peripheral vessels originate independently of the central vessels, 

 in the interior of the vascular layer. They appear as solid cellular 

 branches, which soon become hollow in their interior, and free cells 

 become visible. As these new channels are developed, the omphalo- 

 meseraic vessels gradually diminish and disappear, until at last only one 

 or two pass to the umbilical vesicle. 



Fig. 58. 



FcETAL Circulation : Advanced Period. 



A, Placentulfe ; B', B', Umbilical Veins, with their Common Trunk, B ; D, Vena 

 Portse, and its Anastomosis, C ; E, Ductus Venosus ; F, Posterior Vena 

 Cava ; G, Right Ventricle of Heart ; H, Pulmonary Artery ; J, J, Aorta ; 

 I, Ductus Arteriosus ; K, Umbilical Arteries, with their Anastomosis at the 

 extremity of the ITmbilical Cord. 



The umbilical veins are developed immediately after the formation of 

 the omphalo-meseraic veins, and join these ; as the ramifications of the 

 latter diminish in size the former increase rapidly, and when the liver is 

 formed around them they throw into it those branches which are the 

 rudiments of the hepatic plexus. Between the portal and hepatic 

 veins, the umbihcal vein communicates with the vena cava by the 

 ductus venos^Ls of Aranzi, which does not exist in the foetus of Solipeds 

 in the last month of gestation. The embryonic veins form four chief 

 trunks — two anterior, the anterior cardinal veins ; and two posterior, the 



