466 David M. Davis. 
extending from the left subcardinal into the omphalo-mesenteric 
vein (Plate II). This channel lies in the mesentery, and extends 
in a ventro-mesial direction. It apparently has no relation what- 
ever to the future cava, and seems to be a representative of a more 
or less numerous system of channels in this region. A later stage 
may be seen in Plates III and IV, showing an 8 mm. embryo. 
Here the channels are multiple, and pass from both subeardinals 
into the omphalo-mesenteric vein. One of them is seen in section 
in fig. 1. The one figured here is of almost capillary dimensions. ° 
In this specimen, similar vessels are seen in the mesentery caudal 
to the inferior mesenteric artery, and joining the subcardinal sys- 
tem to the inferior mesenteric vein. AIl these channels appear 
later to be completely obliterated, and take no part in the forma- 
tion either of the vena cava, in the upper part, or of the veins of 
Retzius, in the lower part. Their significance is conjectural, 
although they seem to convey a considerable amount of blood to 
the liver. I have been able to find no previous reference to them. 
It is interesting to note that, in the adult human, veins may pass 
from the duodenum directly to the vena cava, as a rare anomaly. 
The venous changes in the liver during its growth are well de- 
scribed by Hochstetter, whose work has been amplified by recent 
writers. The growth of the liver itself, in the region with which 
we are concerned, is well discussed by Lewis, whose conclusions 
have been referred to. The area designated in Plate III, as ‘‘Site 
of Vena Cava Inferior, ’’is shown as it appeared in section in fig. 2. 
One sees the liver just after it has touched and fused with the 
‘‘caval mesentery.’’ At the top of the drawing are the liver cells 
and liver capillaries, growing downward into the caval mesentery. 
In advance are capillary sprouts—extending out toward other 
capillary sprouts from the somatic area. To understand the 
origin of these second sprouts, another system must be studied. 
In a 7.5 mm. embryo, Plates I and II, the subeardinals, at their 
anterior ends, are still in the growing state. Numerous fine, 
irregular capillaries extend forward, both in and beyond the Wolf- 
fian body. As they pass beyond the confines of the gland, they 
grow into the adjacent tissue, which is, on either side, the abdom- 
inal extension of Ravn’s ala pulmonalis—on the right side we may 
ee) 
