154 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



In injections of the veins of pig embryos, Smith 41 shows definite anasto- 

 moses between the linguo-facial vein and the venous plexuses of the visceral 

 arches. The anastomosis between this vein and the plexus of the first visceral 

 arch is apparently already formed in Ingalls's 4.9 mm. human embryo. In my 

 embryo I have been unable to find an anastomosis, but the direction which the 

 vein takes (plate 5, fig. 1) seems to indicate the possibility that these two sets 

 of tributaries might soon unite. 



VENA CARDINALIS POSTERIOR. 



The blood from the tail and lower half of the embryo is carried to the vena 

 cardinalis communis by means of the vena cardinalis posterior. This arises in the 

 tail of the embryo in the region of the unsegmented mesodermal plates as a slender 

 irregular vessel. In its course to the bend in the back it passes along the ventro- 

 lateral border of the somites, with which it lies in contact. In this part of its 

 course, owing to the plane of sectioning, it is difficult to trace its capillary con- 

 nections, but in places there is evidence that connections similar to those above 

 the nineteenth somite exist below it. 



For the most part that portion of the posterior cardinal vein from the bend 

 in the back to the common cardinal stands out quite sharply. In general it 

 courses along the ventro-lateral border of the somites, lying between them and 

 the pronephros or mesonephros. Opposite the somites of the seventh to eleventh 

 segments the posterior cardinal vein becomes very difficult to follow. It is repre- 

 sented by a slender and apparently solid cord of endothelial cells. In the 

 intersegmental spaces the vessel becomes larger and usually contains a number 

 of blood-cells (plate 5, fig. 1). Opposite the ninth somite (and again the tenth) 

 the existence of the vessel becomes doubtful, owing to the similarity of endothelial 

 to mesenchymal cells. In the corresponding region on the right the continuity 

 of the posterior cardinal vein is even more doubtful. Whether this is due to a 

 closing-up of a once continuous posterior cardinal vein or whether it represents 

 an incompletely developed vein is impossible to determine from the sections. 

 According to Evans 8 , the posterior cardinal veins first appear in human embryos 

 possessing from 15 to 23 somites. He states: 



"It is probable that lateral loops of the dorsal segmental arteries are instrumental 

 in the formation of these veins, as in the case of the anterior cardinals. This method of 

 formation of the posterior cardinal veins appears fundamental. Raffaele (1892) and 

 Hoffman (1893) describe it for selachian embryos and Grafe (1905) and the writer have 

 indicated it in the case of the chick." 



It is probable, therefore, that in my embryo the above-mentioned portions 

 of the posterior cardinal veins are still in the formative stage, particularly since 

 the intersegmental portions of the vein, to which are joined the lateral loops of 

 the segmental arteries, are more definitely marked than the segmental portions. 



From the interspace between the seventeenth and eighteenth segments to its 

 termination, the posterior cardinal vein receives its blood principally from the small 

 lateral loops of the dorsal segmental arteries, as has been described by Evans 8 . 



