A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 155 



In addition to these tributaries, connections can be made out in several places 

 between the posterior cardinal vein and the lateral branches of the aorta, but 

 these connections are not so distinct as the branches from the dorsal segmental 

 arteries. In the regions of the pronephros and mesonephric vesicles small tribu- 

 taries are found arranged in pairs, one lying on either side of these organs; both 

 pass dorsally to join the posterior cardinal vein. The lateral tributaries pass, in 

 the region of the pronephros, between the inner pronephric chambers of one 

 pronephric tubule and the principal collecting tubule of the preceding one. 

 Farther down, in the region of the mesonephros, they pass between the meso- 

 nephric vesicles and the primary excretory duct. 



Evans 8 shows similar vessels in a reconstruction of the 23-somite embryo 

 of Robert Meyer. He also indicates a longitudinal vein which connects the 

 peripheral ends of the medial tributaries and another similarly connecting the 

 lateral tributaries; these he terms the medial and lateral subcardinal veins, re- 

 spectively. I have been unable to make out continuous longitudinal connections 

 in my specimen, but indications of them are apparent on a few of the medial 

 vessels. I find, however, connections between the medial tributary and the 

 lateral segmental arteries, such as Grafe 13 has shown in the chick and Evans 8 

 notes in the Robert Meyer specimen. It is interesting to note that the two 

 above-described tributaries of the posterior cardinal vein are not located with 

 reference to the segments, but (as Evans and others have described for the lateral 

 segmental arteries) they correspond quite closely in number and position with the 

 pronephric tubules, where the latter are present. Below the pronephric tubules 

 they are arranged with reference to the mesonephric vesicles. 



VENA CARDINALIS COMMUNIS. 



The common cardinal vein (duct of Cuvier) is a short, flattened vessel which 

 lies within the sixth body segment. It receives both the anterior and posterior 

 cardinal veins. It is directed caudally in the lateral body-wall and, breaking up 

 into three portions, joins the vena umbilicalis at the point where this vein enters 

 the transverse septum (plate 5, fig. 1, and plate 6, fig. 1). 



VENyE UMBILICALES. 



The umbilical veins begin at the distal end of the body-stalk by the union of 

 several large veins which drain the chorion and its villi. At first they form a 

 single vessel, which soon, however, breaks up into a plexiform arrangement of large 

 veins (plate 5, fig. 1). These reunite to form a single large vein, which again 

 divides to form two smaller vessels, the right and left umbilical veins. Immedi- 

 ately upon separating they pass to the outer border of the umbilical stalk, one on 

 either side, and enter the body-wall. 



In the beginning of its course the right vein is very small. Soon, however, 

 it increases in size and in the remainder of its course it is similar to and about as 

 large as the left umbilical vein, which is quite uniform throughout. Each um- 

 bilical vein, throughout its entire course from the body-stalk to the septum trans- 

 versum, lies within the body-wall, situated in this at about the junction of its 



