I02 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



The posterior cardinal veins develop on each side in the mesonephric ridge, 

 dorso-lateral to the mesonephros (Figs. 103, 104 and 112). Running cephalad, 

 they join the anterior cardinal veins. When the mesonephroi become prominent, 

 as at this stage, the middle third of each posterior cardinal is broken up into sinu- 

 soids (Minot). Sinusoids extend from the posterior cardinal vein ventrally 



Int. car- Ant. car- 

 Metencephalon olid artery dinal vein 



' ' ' \ T/ , I PI, r . 



Mesencephalon 



Ventral aorta 



M\il(nrrplulon 

 I I'h P , 



Ductus 



R. umbilii 



? hord 

 Dc^rt }idiug aorta 



Ph. P. 4 



PuUiiouary artery 

 Linc^uo-facial 

 li in 



R. com. 



cardinal vein 



Sinus venosus 



Com. hepatic 

 vein 



Post, car- 

 dinal vein 



R. subclavian 

 vein 



R. subcar- 

 dinal vein 



Pulmonary 



:cin 



scj;menlal 



Post, cardinal vein Mesonephric arteries 



Fig. 104. — Reconstruction of 7.8 mm. pig embryo showing veins and aortic arches from the left side 

 (after Thyng). X 15. Ph. P. i, 2, 3, 4, Pharyngeal pouches. 



around both the lateral and medial surfaces of the mesonephros. The median 

 sinusoids anastomose longitudinally and form the suhcardinal veins, right and left. 

 The subcardinals lie along the median surfaces of the mesonephroi, more ventrad 

 than the posterior cardinals with which they are connected at either end. There 

 is a transverse capillary anastomosis between them, cranial and caudal to the 



