456 



the V. cava posterior anastomose ventral to the arteries 

 and into the anastomosis the Vv. iliacae internae 

 open ventral to the root of the A. sacralis media. 



The conditions found in the case represented by Fig. 1 are 

 easily explained if we regard the two original Vv. cavae as having 

 fused in the median line ventral to the Aorta. 



A comparison of Fig. 12 with Fig. 18 (Hochstetter's 

 Echidna Embryo No. 45) shows that there is an absolute cor- 

 respondence in each, so far as the mode of formation of the V. cava 

 posterior is concerned. 



In each, the Vv. iliacae internae, after anastomosing 

 with each other ventral to the root of the A. sacralis 

 media, unite Avith the Vv. iliacae externae by means of 

 two vessels that lie dorsal to the A a. iliacae communes. 

 Also ventral to the A. iliaca communis dextra an ana- 

 stomosis exists between the V. cava posterior dextra 

 and the Vv. iliacae internae. 



So far as can be seen by the writer, every single 

 variation in the mode of formation of the V. cava 

 posterior in Didelphys virginiana can be referred 

 back for its explanation, to the conditions described 

 by HocHSTETTER for the development of the V. cava 

 posterior in Echidna. 



For example, in Fig. 4, the ventral anastomosis between the two 

 Vv. cavae has either been lost or was not developed. This prepar- 

 ation is most interesting since it resembles, in all essential details, 

 the conditions met with in an adult Echidna (see Hochstetter's 

 Fig. 16, Semon's Forschungsreisen, Bd. 2, Taf. XVIII). In Hoch- 

 stetter's Fig. 16, if I understand it correctly, the V. cava posterior 

 sinistra unites with the Vv. iliacae internae by means of two vessels 

 that lie dorsal to the arteries in this region. This double union 

 was also found in many instances in Didelphys (Fig. 5). Here, 

 however, the posterior vessel of the two was invariably connected 

 with a vein which I have termed the pudendo-vesicalis ( V.p.v.), since 

 it originates in the region supplied by the artery of the same name. 



In Fig. 5, the union ventral to the aorta or Aa. iliacae communes 

 has been lost, while the union dorsal to the arteries is present only 

 on the left side {D.U.). The dorsal union on the right side having 

 been lost or never having been developed. 



Without entering further into details, I think it will be evident 

 to the reader, after an examination of the figures, that the variations 



