455 



V. cava posterior, as found by him, in two embryos of Echidna 

 aculeata^). 



In the Echidna Embryo No. 44 (Fig. 17) the V. cava posterior 

 consists, caudad of the Hver, of an unpaired and a paired division. 

 The unpaired division extends from the liver on the right side of 

 the A. omphalo-mesenterica, to a point somewhat caudad of the 

 origin of this artery. From this point the two Vv. cavae extend 

 caudad, at first lying ventral and then on either side of the Aorta 

 and subsequently anastomose in the median line ventral to the 

 Aorta directly behind the A. mesenterica inferior. The two Vv. 

 iliacae internae which, as Hochstetter says, must be regarded as 

 continuations of the two Vv. cavae, open into the above mentioned 

 anastomosis, ventral to the Aa. iliacae communes. 



^In the Echidna Embryo No. 45 (Fig. 18), the unpaired division 

 of the V. cava posterior possesses the same relations as in Embryo 

 44. In the paired division, however, there is a difference. Here the 

 anastomosis of the two Vv. cavae, caudad of the A. mesenterica 

 inferior, no longer exists. Also, the two Vv. iliacae internae unite 

 with each other in the median line, ventral to the root of the 

 A. sacralis media and then, by means of two vessels that pass 

 dorsal to the Aa. iliacae, anostomose with the V. iliaca externa 

 dextra and sinistra, respectively. 



The anastomosis that lies ventral to the Aorta in Embryo 44, 

 between the Vv. iliacae internae and the two divisions of the V. cava 

 posterior is slightly changed in Embryo 45. Here it exists only 

 between the V. cava posterior dextra and the Vv. iliacae internae. 



In comparing the conditions met with in Didelphys with those 

 found in the Echidna embryos, let us first consider the general 

 relations of the V. cava posterior. 



In the first place, the large percentage of bifurcated Vv. 

 cavae or indications of the same, in the form of foramina met 

 with in Didelphys make it appear , as not improbable , that 

 the V. cava of Didelphys may at a certain stage of its devel- 

 opment consist, as in Echidna, of an unpaired and a paired 

 division. In Fig. 3 these two subdivisions are present and the con- 

 ditions, so far as the anastomosis of the veins ventral to the 

 arteries is concerned, are essentially the same as those in the 

 Echidna Embryo 44 (Fig. 17). In Fig. 3 the two divisions of 



1) Hochstetter in Semon, Zoolog. Forschungsreisen in Australien 

 etc., Bd. 2, Lief. 3, Monotremen und Marsuj)ialier. 



