THE MAMMALIAN VENA CAVA POSTERIOR 11 



It is evident, for the most part, that in the 16-mm. cat em- 

 bryo the veins of the lumbar region still retain their early bi- 

 laterally symmetrical plan a condition which later becomes 

 transformed into an asymmetrical one. This change is brought 

 about chiefly by the loss and atrophy of vessels on the left side. 

 For example, the vena cava posterior of the adult cat is de- 

 rived from the following embryonic veins : the right lumbar 

 supracardinal (Sprc.Dext., B), which retains, through a dor- 

 sal anastomosis, a connection with the left iliac vein; the 

 right subcardino-supracardinal anastomosis (Sitbc.Sprc. 

 Anast., the right side of the renal collar) ; the right subcar- 

 dino-posterior cardinal anastomosis (Subc.Pc.Anast.) ; the 

 intersubcardinal anastomosis (Int.Svbc.Anast.) ; a portion of 

 the right subcardinal, the pars subcardinalis (P.Subc.), and 

 the V. hepatica communis and the hepatic sinusoids (pars 

 hepatica, P. Hep.). The lateral portion of the renal collar on 

 the left side and the cranial half, at least, of the left lumbar 

 supracardinal (fig. 3) undergo atrophy; but the caudal half of 

 the left lumbar supracardinal appears to fuse with the right 

 to form the caudal half of the adult caval vein. 



With the atrophy of the left side of the renal collar (Sub<-. 

 Sprc.Anast., fig. 3), the left renal vein (R.V.) retains its con- 

 nection with the left subcardino-posterior cardinal anasto- 

 mosis (Subc.Pc.Anast.), which forms the proximal portion of 

 the adult left renal vein. Into the latter the left posterior 

 cardinal (Pc.Sin.,D, fig. 3), with its gonad tributaries, opens, 

 thereby establishing the adult condition in which the internal 

 spermatic vein joins the left renal vein. On the right side 

 of the embryo (fig. 2), the right embryonic renal vein (R.V.) 

 retains its original connection with the right side of the renal 

 collar (Subc.Sprc.Anast.), which forms a segment of the adult 

 caval vein. The right posterior cardinal (Pc.Dext.,A), witli 

 its gonad tributaries, also retains its connection with the renal 

 collar at its ventrocaudal border, and owing to an elongation, 

 in a craniocaudal direction, of the right side of the collar 

 (Subc.Sprc.Anast.), the junction of the posterior cardinal 

 with the cava is carried somewhat caudal to that of the right 



