DEVELOPMENT OF THE VEINS IN THE EMHKYo IMC. 21 



in the sagittal plane passing lateral to the spinal ganglia. The spinal veins collect the blood 

 from an exceedingly abundant capillary plexus around each ganglion, and the vein in each 

 case corresponds to the cephalic surface of the ganglion. This point is clear in figure 4, from 

 a pig 7 mm. long, in my articles of 1913. Thus, from the edge of each spinal ganglion a 

 short, stout vein passes ventralward to the posterior cardinal vein (fig. (>) in a plane corre- 

 sponding to the edge of the body-cavity and distinctly farther lateralward than the posit ion 

 of the future azygos system. 



While the primitive spinal arch is being elaborated, the next step is being made 

 namely, the second vascular arch of a typical body-segment is being formed. As can be 

 seen in figure 6, a tuft of arteries from the root of the dorsal segmentals or the adjacent 

 part of the aorta itself grows to a third longitudinal capillary plexus, which extends through- 

 out the body and lies lateral to the myotomes. This capillary plexus, which is shown for 

 the neck region in figures 4 and 5 of my articles of 1913, is very characteristic. It extends 

 from the base of the skull to the tip of the tail. Thus the three sets of arteries give rise to 

 longitudinal chains of capillaries: the dorsal segmental arteries to the pial capillaries, the 

 lateral arteries to the cardinal system, and the third set of dorso-lateral arteries to the 

 capillary plexus along the myotomes. These capillary plexuses are exceedingly significant. 

 For example, the primary capillary plexus along the dorso-lateral surface of the cord lies 

 along the line in which the nerve-cells are multiplying to form the spinal ganglia; secondly, 

 the longitudinal chain from the lateral arteries develops in response to the tubules of the 

 Wolffian body; and, thirdly, the superficial dorso-lateral plexus in response to the myo- 

 tomes. Throughout the study of the evolution of the vascular system the pattern of the 

 sheets of capillaries is each time determined by the development of the tissues. The lateral 

 vascular arch is not at first a segmental one, for the vein is the umbilical vein. The relation 

 of the capillary plexus along the myotomes to the umbilical vein is shown in figure 6. The 

 lateral arch, then, consists of dorso-lateral arteries which lead through a longitudinal chain 

 of capillaries, along the lateral surface of the myotomes, into veins which are branches of 

 the umbilical vein. In this vascular pattern the umbilical vein is destined to be replaced 

 first by the posterior cardinal vein and permanently by the thoraco-epigastric, the internal 

 mammary, the transverse lumbar veins, and the azygos system. 



Two other points of interest may be noted in figure 6, namely, that there are anasto- 

 moses between the capillary plexus lateral to the myotomes and the capillary plexus of the 

 cord, and that there is a tiny vein from the myotome-plexus to the spinal vein, so that 

 these two primary vascular arches are already connected at this stage. 



To sum up the circulation of the body-wall at the stage of 10 mm., there are in general 

 two vascular arches, a spinal and a lateral arch. The spinal arch is segmental and consists 

 of spinal arteries which lead through three capillary plexuses (namely, the plexus of the 

 lateral surface of the cord, of the ventral surface, and of each ganglion) into a series of seg- 

 mental veins which connect with the cardinal vein of the Wolffian body. The lateral arch 

 consists of dorso-lateral arteries which drain through a longitudinal capillary plexus lateral 

 to the myotomes, into a series of non-segmental branches of the umbilical vein. 



The next stage in the development of the veins of the body-wall is the beginning of the 

 thoraco-epigastric vein, as has been shown by Miss Smith. It is a longitudinal trunk which 

 develops in the capillary plexus of the body-wall and joins the primitive ulnar vein when the 

 embryo is about 12 mm. long. The thoraco-epigastric vein thus precedes the azygos system 

 in development, for the latter does not begin until the embryo is about 14 mm. long. The 

 position of the thoraco-epigastric vein with reference to the body-wall is plain in figure 

 8, plate 3; figure 10, plate 4; and figure 13, plate 6. It lies just ventral to the capillary 



