DEVELOPMENT OF THE VEINS IN THE EMBRYO VIC,. 2'.} 



fifth segment caudalward the spinal veins are in a continuous sheet of vessels distinctly 

 lateral to the plexus which is the anlage of the azygos. In this continuous plexus of vessels 

 which enters the Wolffiau body at its dorso-median angle, the spinal veins stand out only 

 because they are a little larger than the rest. This lateral sheet of capillaries and veins 

 is destined to develop very much and to become so important that it should be emphasized 

 from the beginning (fig. 12). It indicates that the veins of the body-wall are far more 

 numerous than the segmental veins. The subvertebral capillaries represent a t hird vascular 

 arch for the body-wall, the first being the spinal, the second the lateral, and the third the 

 subvertebral. 



The first figure showing a definite azygos vein which develops out of this capillary 

 plexus is seen in a dissection in figure 9, plate 4. The left Wolffian body, the entire left pos- 

 terior cardinal vein, and hemiazygos vein are shown in place. The subvertebral plexus is 

 much more highly developed, especially in the cervical region, and in the capillary plexus 

 many definite veins are clearly seen. It is important to note that both the prevertebral 

 cervical plexus and its continuation in the dorsal region have many more connections with 

 the cardinal system than there are segments. The azygos system develops out of that part 

 of the prevertebral system which is in the body region, and of the many connections with the 

 cardinal vein that which is nearest the duct of Cuvier becomes the permanent one. This is 

 true for both sides, and in the pig the duct of Cuvier is permanent on both sides. In the 

 stage represented by figure 9 the duct of Cuvier is high up in the thorax, so that the vessels 

 representing the accessory hemiazygos are a very short group connecting abundantly with 

 the cervical plexus. In general the length of the azygos and of its corresponding superior 

 intercostal is determined simply by the position of the duct of Cuvier. The lower end of 

 the hemiazygos at this stage is opposite the third or fourth dorsal vertebra, where the vein 

 breaks up into a subvertebral capillary plexus. 



The azygos system which begins simply in response to the developing vertebrae becomes 

 larger, because it establishes connections with the spinal veins and their new tributaries, 

 intercostal veins. Moreover, this connection with the spinal veins takes place as the pos- 

 terior cardinal system is becoming less important through being limited to the anterior part 

 of the Wolffian body. 



These new relations of the azygos system can be understood by comparing figure 13, 

 plate 6", and figure 10, plate 4, one a dissection of a total specimen, the other a transverse 

 section of approximately the same stage. It has been made clear that the azygos begins as a 

 prevertebral plexus and that its marked increase in size is due to its secondary relations to 

 the spinal and intercostal veins. By comparing figures 6 and 10 it will be seen that there 

 has been a marked development of the veins of the body-wall as well as of the spinal veins. 

 Figure 10 is a section of an injected embryo pig, 16 mm. long, cut transversely 250 M 

 thick. The section passes through the anterior pole of the Wolffian body and shows that 

 part of the organ which drains throughout its history into the posterior cardinal vein. It is 

 well above the point of entrance of the vena cava into the liver (compare figure 2) and its 

 veins do not drain into the vena cava. The posterior cardinal vein and the ventro-lateral 

 vein are both shown. In the median position is the short median trunk, the anterior 

 median vein shown in figure 2 for an earlier and in figure 4 for a later stage. In the series 

 from which figure 10 is taken this vessel is short, measuring not more than 0.7 mm. It 

 ends below by anastomosing with the esophageal vessels; above it joins the ventro-lateral 

 vein and thence the posterior cardinal, figure 4. These short anterior median trunks are 

 constant and can be found as long as the posterior cardinal veins persist. They lie in part 

 ventro-lateral and in part directly lateral to the aorta. They anastomose with each other 



