Superficial Veins in the Pig. 441 



cord. The posterior cardinal and the umbilical veins share in drain- 

 ing the limb buds, one dorsally, the other vcntrally, and the umbilical 

 drains as well a plexus which forms in the membrana reuniens. 

 As the posterior cardinal vein sinks into the depth, though it con- 

 tinues to partly drain the limb buds and to receive its segmentals 

 from the myotomes, the relatively vast area of the membrana re- 

 uniens drains entirely into the umbilical vein. Then, as the plexus 

 in the membrana increases in complexity and the muscle layer 

 shifts into it ventrally, larger longitudinal anastomoses form along 

 the body wall draining up under the anterior limb bud to the ves- 

 sels there, which connect in turn with the cardinal veins. Soon a 

 definite vessel is formed, the thoraco-epigastric, which increases rap- 

 idly in size until it collects to itself almost all the tributaries of the 

 umbilical vein. The plexus in the membrana reuniens, in conse- 

 quence, gradually dies out until only a few vessels in the median line 

 and in the lower ventral region remain. Meanwhile on the mesial 

 side of the muscle layer and ribs, other vessels, namely, the in- 

 ternal mammary and the deep epigastric veins and arteries have 

 been formed from a longitudinal plexus, and the intercostal vessels 

 have been spun out like strands of cobweb in the intercostal spaces. 

 All these vessels not only anastomose among themselves, but also 

 have very numerous communications with the superficial vessels. 

 Finally, it comes about, by changes given in detail later, that the 

 thoraco epigastric loses its axillary connections, and drains into the 

 internal mammary. After this the vessels of the body wall are not 

 changed in kind, but in degree only, and the condition of the adult 

 is practically achieved. 



Turning to the literature concerning the development of the super- 

 ficial blood vessels of the body wall, we find a number of observa- 

 tions upon the subject, but these are given, in great part, either 

 as isolated or unexplained steps, or in an attempt, more or less 

 satisfactory, to exjDlain the condition in the adult. These latter 

 observations, for the most part, are made upon fcetuses, and so only 

 a clue can be gleaned here and there concerning the early devel- 

 opment. Among observations upon very young embryos are those of 

 Coste, Kolliker, His, and Mall. 



