ON THE ARRANGEMENT OF THE FCETAL MEMBRANES IN THE OETACEA. 481 



and contained foetus could be examined. The chorion itself was seen to be 

 distinctly divided into two layers ; the outer villous, already described, and an 

 inner thin translucent membrane. These layers were attached to each other 

 by very delicate connective tissue, and between them, the chorionic arteries 

 and veins, passing to and from the villi, ramified. 



As the chorion showed no trace of putrefaction, I decided to make an 

 injection of the umbilical vessels and, with the skilful co-operation of my 

 assistant, Mr Stirling, have succeeded in obtaining some beautiful prepara- 

 tions in illustration of the vascularity of the fcetal membranes/" 



The umbilical cord, 15 inches long, consisted of two arteries, two veins, and 

 the pedicle of the allantois (urachus), which were held together by an areolated 

 connective tissue, and were invested by the amnion (Plate XVII. fig. 7, a, and 

 Plate XVIII. fig. 15). A careful search was made in the substance of the cord 

 for the umbilical vesicle, or its pedicle, and for the vessels of the vitellus, but 

 without any positive result ; for although some elongated threads, which could 

 be isolated from the surrounding tissue, were met with, yet their impervious 

 condition prevented one from concluding with any certainty that they were the 

 remains either of the vitelline duct or of its accompanying blood-vessels. 



The two umbilical veins resulted from the bifurcation at the umbilical 

 aperture of the single intra-abdominal vein. They were of large size, and 

 placed at the sides of the cord ; and the urachus, with the two umbilical arteries, 

 was situated between them. Sixteen inches from the abdomen of the foetus 

 the cord bifurcated into a right and left branch, an artery and vein passing off on 

 each side, conducted by the allantois, and situated between that membrane and 

 the amnion, to the chorion. In their course the artery wound around the vein 

 in a spiral manner from left to right on the one side, from right to left on the 

 other. Ten inches from the angle of divergence the vessels reached the chorion, 

 along the line of attachment of the allantois to that membrane, and in their 

 course gave off several branches. As soon as they reached the chorion, the 

 branches of the vein diverged from the corresponding arteries, each pursuing 

 an independent course, and ramifying in an arborescent manner between the 

 two layers of the chorion. The general mode of branching was dichotomous, 

 but occasionally, and this more especially with the arteries, collateral branches 

 arose, in which case two not unfrequently came off close together, and extended 

 for some distance side by side before they proceeded to their respective areas 

 of distribution. Several anastomoses were observed between the branches of 

 the larger veins, and the finer branches of the arteries were occasionally 

 observed to inosculate with each other. 



The mode of origin of the terminal arteries varied with the arrangement of 



* These and the other preparations obtained from this uterus are preserved in the Anatomical 

 Museum of the University of Edinburgh. 



