398 ALEXANDER S. BEGG 
relation to the left atrium. At the atrioventricular groove the 
vein turns to the right, and after receiving the cardiac veins 
empties into the right atrium. The splenic vein and the hemi- 
azygos are in rather close proximity, being separated by the crus 
of the diaphragm, and while no connecting vessel was observed, 
capillary connection would not seem improbable. 
Heretofore the anomalous absence of the inferior vena cava 
has been found either after birth or in older embryos, at a time 
when the abnormal condition has been long established. The 
specimen here to be described is a pig embryo of 12 mm., a stage 
not much later than that in which the anastomosis between the 
subeardinals and the hepatic sinusoids first occurs. The ar- 
rangement of vessels so closely resembles the condition normally 
found at an earlier period that one questions, at first examination, 
if indeed anything is amiss, but a study of other embryos in the 
Harvard Collection shows that the formation of a vena cava in- 
ferior has begun in 6-mm. specimens and is well advanced in 
those of 7.5 mm. As is well known, a large vessel should be 
present at 12mm. The development of the various organs cor- 
responds with that in other 12-mm. embryos, and the presence 
of an associated and unquestioned anomaly in the portal system 
of veins confirms the correctness of the recorded measurements. 
In addition to these considerations, the density of the mesen- 
chyma investing the liver, as well as that within the caval mesen- 
tery, leads to the conclusion that this specimen would never 
have possessed a vena cava inferior. 
The cardinal system of veins with adjacent venous channels 
has been modeled in the abnormal specimen and also in a normal 
embryo of like size for comparison. These models were made in 
wax and plated with copper, following Wallin’s method. While 
the cardinal and the veins of the Wolffian bodies of both sides 
were modeled, only the right side is shown in the drawing. 
The appearance of the veins as shown by the model of the nor- 
mal embryo is that made familiar by the graphic reconstructions 
published by Lewis (’03) and by the cleared injected embryos 
exhibited by Professor Sabin at the Philadelphia meeting of the 
American Association of Anatomists in 1913 (afterward pub- 
lished, 1915). The vena cava inferior is seen to have reached a 
