24 DEVELOPMENT OF THE VEINS IN THE EMBRYO PIG. 



freely ventral to the aorta, and are connected with the esophageal veins, with the veins 

 of the Wolffian body, and with the azygos and hemiazygos veins. I think them important 

 because they account so readily for variations in the azygos veins, such as the one reported 

 by E. R. Clark (1915). 



Turning now to the azygos veins themselves at this stage, namely, 16 or 17 mm., the 

 veins are gradually extending caudalward and for about 5 segments have taken over the 

 spinal and intercostal veins (fig. 13). At the same time it will be noticed that two new 

 longitudinal veins, namely, the internal mammary and the thoraco-epigastric veins, are hot li 

 larger than the azygos vein, and thus carry much the largest part of the blood from the 

 body-wall back to the heart. For the upper thoracic segments, however, the azygos vein 

 now takes much of the blood that formerly passed through the Wolffian bodies, there being 

 only small connections with that organ. Below the level of the fifth segment, as seen in 

 figure 13, the spinal veins and the abundant plexus of the body-wall form a continuous sheet 

 of vessels which enter the Wolffian body lateral to the line of the azygos vein. 



Certain relations of the azygos system are only to be made out in sections. As seen 

 in figure 10, the subvertebral plexus is much further developed. There is now a capillary 

 plexus of fine capillaries, mesial to the spinal arteries, resting on the bodies of the vertebra?, 

 while lateral to the spinal arteries is a coarser plexus in which lie the azygos veins. This 

 lateral plexus drains not only the subvertebral plexus, but connects with the spinal veins 

 and with the exceedingly complex plexus from the body-wall as well. 



In figure 10 will be noted a space just between each azygos vein and the small vessels 

 from the plexus which enter the Wolffian body just at the edge of the body-cavity in the 

 position of the original spinal veins. In this space runs the sympathetic chain, and thus it 

 is clear that by means of the subvertebral plexus the spinal veins have been shifted from a 

 position lateral to the sympathetic to the azygos veins which are medial to the sympathetic. 



From figures 10 and 13 one can readily see that the circulation of the body-wall has now 

 become exceedingly complex as compared with the two simple arches of the stage of 10 mm. 

 Moreover, the pattern is very definite and is repeated for segment after segment. The 

 pattern of each segment is only to be analyzed by following the pattern of the capillaries 

 with reference to the developing tissues, the ribs, the intercostal muscles, and the mus- 

 culature of the back. 



The shifting of the spinal veins to the azygos vein is most plainly shown in figure 11, 

 from an injected embryo 20 mm. in length. Here there is a very definite azygos vein which 

 is shown passing beneath the right posterior cardinal vein. By means of the plexus shown 

 in figure 10, the second, third, and fourth spinal veins, with all the plexus between, have 

 clearly shifted to the azygos vein. The fifth spinal vein still enters the Wolffian body in 

 the more lateral line, while the sixth runs to the azygos vein. The azgyos vein ends in 

 capillaries opposite the seventh segmental vein, which enters the Wolffian body directly. 

 Thus it is clear that the azygos system receives at the stage of 20 mm. about 7 of the seg- 

 mental veins. 



The relations of the rest of the spinal veins at this stage are exceedingly interesting. 

 It will be noted in figure 1 1 that the eighth and ninth join and run to the level of the tenth, 

 where this common stem enters the Wolffian body in the primitive lateral line. This is 

 simply an anomaly of this particular specimen and without especial significance. The 

 position, however, of the tenth, eleventh, the combined stem of the twelfth and thirteenth, and 

 the fourteenth is extremely interesting. From the primitive lateral position in which they 

 entered the dorsal vein of the Wolffian body these vessels have shifted to a more medial 

 position. Their relations can only be analyzed by means of sections. These spinal veins 



