464 



Helen Williston Smith 



paired segmental arteries are seen passing out at a wide angle from 

 each other, and each breaking up into a tuft of vessels, twigs of which 

 run into the inner and outer side of the muscle layer and to the an- 

 terior spinal artery and cord. These twigs pass into fine capillaries 

 which may then be traced into the veins. The venous blood is car- 



FiG. 6. — Section one-half millimeter thick of an emhryo 15i/^ millimeters 

 long. 



MS, spinal cord.; V.ji, internal jugular vein ; 'T, trachea ; Aa, fourth aortic 

 arches; MR, membrana reuniens. 



ried oif in one of three ways. Either it may run through the thoraco 

 epigastric to the posterior cardinal, or it may run into the membrana 

 to the umbilical vein, or it may run back through the dorsal seg- 

 mental veins into the mesonephros to the posterior cardinal veins 

 (Vcp). The sections of this embryo also show that at this stage the 

 internal mammary vein and artery are not present, as such, but that 



