116 
JANE I. KOBERTSON. 
immediately internal to them, the outer and inner channels of 
the posterior part of each posterior cardinal vein are sepa- 
rated to some extent. Later the inner trunks meet and fuse 
temporarily, forming a median interrenal vessel (Text-fig. 
24 E, I. R. F.), while the outer trunks form the renal portal 
vessels (Text-fig. 24 e, J?. 2-^.). In front of the mesonephros 
the outer vascular channels disappear, becoming apparently 
fused with the inner channels. Numerous anastomoses occur 
between the inner and outer trunks through the substance of 
the mesonephros. 
The development of the posterior cardinal veins up to this 
point thus closely resembles that of the same vessels in the 
Urodeles ( 11 ), with this difference, that in Lepidosiren the 
fusion of the two posterior cardinal veins across the middle 
line is only a temporary, not a permanent condition. 
At the same time (Stage 31) changes occur in the posterior 
cardinal veins cranial to the mesonephros. The right pos- 
terior cardinal becomes the larger of the two vessels, and trans- 
verse anastomoses appear segmentally between them as far 
forwards as the region of the pronephric glomeruli (Text-fig. 
24 E, r. and /. P. Card .) . The main blood-stream is thus deflected 
to the right and a large new vascular channel appears in the 
mesentery of the liver. This new channel arises on the right 
at the level of the most anterior anastomosis between the 
posterior cardinal veins (Text-fig. 24 e,P. F. (7.), and passes 
forwards in the triangle formed by the pronephros on the 
right, the lung rudiment on the left and the yolk and liver 
ventral ly. Where it passes from the surface of the yolk on 
to that of the liver, the anterior termination of this vessel is 
continued into the wide channel formed by the liver sinuses 
and subintestinal vein and so enters the sinus venosns (Stage 
31), while posteriorly it is in continuity with the wide trunk 
of the right posterior cardinal vein, thus forming the pos- 
terior vena cava. The front part of the posterior vena cava 
appears in the mesentery of the liver, on the ventral sur- 
face of the lung rudiment ventral to the common pnlmouai-y 
vein. This close relationship is maintained throughout 
