22 DEVELOPMENT OF THE VEINS IN THE EMBRYO PIG. 



plexus of the body-wall and lateral to the ventral border of the myotomes. It is not in 

 the thin lateral wall which contains the veins to the umbilical vein. 



ORIGIN AND DEVELOPMENT OF THE AZYGOS SYSTEM AND THE LOWER SEGMENT OF THE INFERIOR 

 VENA CAVA FROM A PREVERTEBRAL PLEXUS. 



From this pattern of two vascular arches, a spinal and a lateral as a basis, the azygos 

 veins must be followed. It will be noted in figure 6 that there are no vessels between the 

 aorta and the ventral spinal plexus, that is, the area around the notochord is a non-vascular 

 area; moreover, this remains a non-vascular area until the bodies of the vertebrae begin to 

 differentiate. This differentiation begins in the neck region, and by the time the embryo 

 is 1 1 mm. long there is an abundant prevertebral plexus in the neck. This is clearly shown in 

 figure of my articles of 1913. In figure 7, plate 3, a dissection of an injected embryo 14 

 mm. long, it will be seen that the prevertebral plexus of the cervical region extends into 

 the dorsal region. This figure represents a ventral view of an embryo from which the 

 heart, left Wolffian body, and left posterior cardinal vein have been removed. The right 

 Wolffian body is shown as solid, since the India-ink injection made it so black as to pre- 

 clude any analysis of its veins. 



The cervical and upper dorsal prevertebral plexuses join both thecardinal veins and 

 their tributaries, the spinal veins, in many places. Between these two chains of capillaries 

 are the bodies of the vertebra', so transparent in the specimen that the anterior spinal plexus 

 shows faintly in the drawing. The double chain of capillaries, which is the anlage of the 

 azygos system, shows in cross-section for about the same stage in figure 8, from an embryo 

 13.5 mm. long, the specimen being slightly further advanced than that of figure 7. This 

 section passes through the upper half of the Wolffian bodies, where the organ is wide (see 

 fig. 7), and shows within the Wolffian bodies only the transverse veins. On the right 

 side of the figure the spinal artery joins the ventral spinal plexus; on the left, the section 

 passes through the full length of a ventral spinal artery to the anterior spinal artery. On the 

 left side the lateral spinal artery is clear. On neither side is the full course of a spinal vein 

 shown, for the section passes through the intersegmental plexus; but on the right side a 

 partial view of a spinal vein indicates their position. The point at which the spinal veins 

 enter the Wolffian body is very clear on both sides, mesial to the edge of the body-cavity. 

 Ventro-lateral to the bodies of the vertebra, along the spinal arteries, is a capillary plexus 

 draining by many branches into the border of the Wolffian bodies, just lateral to the aorta. 

 The section shows no indication of an azygos vein merely the antecedent capillary plexus. 

 It is important to notice the numerous connections in every segment of this capillary plexus 

 with the vessels of the Wolffian body. 



This section represents well the point of the change in the body-wall circulation due 

 to the development of the thoraco-epigastric vein; it shows a vein still draining the myo- 

 tome-plexus into the umbilical, but the new thoraco-epigastric vein lies in the body- wall 

 and makes a very important chain in the lateral vascular arch. The group of the lateral 

 arteries to the myotome-plexus is very plain. The section shows that the thoraco-epigastric 

 vein precedes the azygos system, for the former is a large vein, while the azygos is still a 

 capillary plexus. In this cross-section it can be seen that the sub vertebral capillary plexus 

 lies medial to the spinal vein. Its arteries are tiny branches of the spinal arteries which are 

 best seen in the injections of silver nitrate. On the left side of this section three small veins 

 connect this plexus with the border veins of the Wolffian body just lateral to the aorta. The 

 sympathetic chain lies between the subvertebral or azygos plexus and the spinal veins. 



As can be seen in figure 7, the azygos plexus at this stage, namely, at 14 mm., extends 

 only three or four segments into the thoracic region. In this figure it is clear that from the 



