Circulatory System 



75 



Fig. 75. Diagrams illustrating stages in the development of the systemic veins 

 of the pig. The cardinal and omphalomesenteric veins are shown in black, the sub- 

 cardinal system is stippled, the supracardinals are horizontally hatched, and vessels 

 arising independently of these three systems are indicated by small crosses. [A) 

 Ground plan of the veins of a young mammalian embryo. (B) Cross section (al 

 level of arrow in A) showing dorsoventral relations of the various veins (C) dia- 

 grammatic plot of veins of 5-6 mm. pig embryos. (D) Arrangement of veins in 



