YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 29 



the first somite, whore it turns out laterally into the anterior cardinal. The vena capitis 

 medialis lies throughout its course close in against the side of the brain and passes ventro- 

 medially to the trigeminal, acusticofacial, glossopharyngeal, and vagus ganglia and the 

 otocyst. It lies so close against the trigeminal ganglion, whose under surface is irregular, 

 as to give the appearance of penetrating it. In embryos described by Ingalls (1907), 

 Broman (1896), and Mrs. Gage (1905), which are somewhat older than this embryo, the 

 vein lies lateral to the acusticofacial ganglion. Mall (1905) states that the first position 

 of the vein is medial to the ganglia, but that loops form around them and the medial branch 

 disappears, leaving the vein lateral to all but the trigeminal. This process has not yet 

 begun in this embryo of 18-19 somites, but is completed in Mrs. Gage's of 27 somites. 



The anterior cardinal vein (plate 1 and plate 3, fig. 4) is the direct continuation of the 

 vena capitis medialis, where the latter turns out in front of the first somite. The anterior 

 cardinal vein lies lateral to the somites and on the right side extends to the middle of the 

 third, on the left to the beginning of the third somite, where it ends in the duct of Cuvier. 

 The finding of the duct of Cuvier at this level is evidently normal for early stages, as Evans, 

 in the account of the vascular system in Keibel and Mall's Human Embryology, places it 

 here, and Williams (1910), in chick embryos of 15 and 18 somites, also found it at this level. 



The posterior cardinal vein (plate 1 and plate 3, fig. 4) is as yet very short, and can 

 only be traced back from the duct of Cuvier to the sixth somite. Beyond this it is not 

 recognizable, though isolated spaces, apparently vascular, occur along the line of its future 

 course. This is the youngest embryo in which the posterior cardinal vein has yet been 

 found. It is only a very short trunk in the embryo of 23 somites described by Thompson, 

 and in Bremer's embryo of 4 mm. it is only a very small bud situated on the duct of Cuvier. 



The vitelline veins (plate 1) commence in a plexus over the yolk sac and run forward 

 in the wall of the yolk sac near the body wall and parallel to the long axis of the body. At 

 the anterior surface of the yolk stalk they turn upward into the body, being situated in 

 the septum transversum and immediately lateral to the pleuroperitoneal passage. Just 

 in front of the opening of this passage into the ccelom each unites (plate 3, fig. 4) with the 

 umbilical vein of the corresponding side. There are no veins or anastomosing channels 

 uniting the vitelline veins of the two sides anteriorly; their only communication here is 

 through the sinus venosus, but such communications occur, Low (1908) describing one in 

 an embryo of 14 somites. 



The umbilical veins, two in number, start in the chorion and enter the belly stalk, 

 running in its dorsal portion (plate 1), and as the belly stalk broadens out, they lie in its 

 dorsal angles just under the reflection of the amnion from it. In the belly stalk the veins 

 show frequent anastomoses, forming a blood sinus similar to that described by Dandy 

 (1910), and finally separate to enter the body, one on each side. They run forward in the 

 somatopleure, forming a bulging ridge under the line of reflection of the amnion. On 

 reaching the septum transversum each unites with its respective vitelline vein into a large 

 common trunk (plate 3, fig. 4) which receives the diminutive duct of Cuvier, as they all 

 pass into the sinus venosus. 



The description of the veins of Embryo V is essentially similar to the above. The 

 posterior cardinal vein can not be so distinctly followed, however, and there is an interval 

 in front of the first somite where the vena capitis medialis can not be distinguished. These 

 veins, however, may be present, for vessels in this embryo are hard In follow, and serial 

 sections, it is well known, do not give the best results unless the vessels have been previously 

 injected. 



