Superficial Veins in the Pig. 453 



There are still many blood vessels in the membrana (MR), but 

 they show a tendency towards atrophy. The thoraco epigastric 

 (Vte), on the other hand, which in the previous figure (Fig. 4) 

 was nothing but a thread of capillaries, in Fig. 5 shows as a very 

 distinct vessel passing up under the limb bud, receiving the prim- 

 itive ulna, and running on up to enter the posterior cardinal at its 

 junction with the anterior cardinal. The central connections of 

 the thoraco epigastric, in this figure, show very well, but the picture 

 is not always so uncomplicated. It will be noticed that the vein runs 

 dorsal to the artery while in the adult it is normally ventral. The 

 change is effected as follows. There is at the root of the arm bud a 

 capillary mesh surrounding the artery connected above with the car- 

 dinals at their junction and below with the jtrimitive ulnar. In 

 young stages the dorsal portion of this capillary mesh or loop is often 

 the more prominent, but, later, the blood tends to take the ventral 

 short cut and the dorsal part astrophies. The thoraco epigastric 

 may be looked upon as a continuation of this mesh extending out 

 upon the body wall. It drains a small area, below and dorsal to the 

 limb bud and has numerous connections with the capillaries supply- 

 ing the lower part of the membrana reunions. These latter capil- 

 laries, it will be noticed, show a tendency to form into a long rope- 

 like plexus, running from the region ventral to the posterior limb bud 

 up towards the anterior limb bud. This rope of capillaries later 

 forms the superficial epigastric (Ves) and may therefore be consid- 

 ered as part of the permanent type. 



That the injection over the heart of this embryo is not complete 

 is very probable because sections of this same stage show injection 

 all over the upper heart region. This is shown in Fig. 6. Here, on 

 either side of the U-shaped aortic arches, we find good-sized veins 

 that empty into the internal jugulars (Vji). These veins drain the 

 upper part of the membrana over a bib-shaped area, somewhat greater 

 than is shown in the section, and anastomose on the sides with veins 

 that enter the posterior cardinal at its junction with the anterior 

 cardinal. Fig. 7 is from a section of this same embryo at the level 

 of the omphalomesenteric artery and shows the way in which the 

 blood is supplied to the muscle layer and to the body wall. The 



