ANATOMY OF A 17.8 MM. HUMAN EMBRYO 87 



sinus-like channels, and in 1900 he introduced additional histo- 

 logical evidence of the presence of these channels to which he 

 applied the name 'sinusoids.' It is also noteworthy that Hoch- 

 stetter ('93) expressed doubt whether in man the azygos and 

 hemiazygos veins could be considered in their entirety as rem- 

 nants of the posterior cardinals. 



Only the right posterior cardinal derivatives have been recon- 

 structed (plate 4) . 



The middle sacral component (V.s.m.) of the right posterior 

 cardinal begins in the tail-process and extends cephalad ventro- 

 lateral to the arteria sacralis media (A.s.m., pi. 1), receiving as 

 tributaries the right caudal intersegmental veins. 



At the level of the fourth and fifth lumbar intersegmental veins 

 the right vena sacrahs media is connected to its companion vein on 

 the left by a relatively large, transverse anastomosis {x'), passing 

 ventral to the middle sacral artery. Other anastomoses of the 

 middle sacral veins exist more caudally. 



It is by virtue of this large transverse anastomosis (which 

 becomes the terminal part of the left common iliac vein of the 

 adult) that the left middle sacral and the veins of the left lower 

 extremity, come to drain into the inferior vena cava. 



Two venous channels (sub- and supra-ureteral) extend cephalad 

 from the right middle sacral, enclosing between them an area of 

 mesenchyma through which passes the ureter ( Ur.) . The smaller, 

 sub-ureter al channel having at first passed dorsal to the Wolffian 

 body, with the sinusoids of which it is intimately connected, 

 bends medially to terminate in the caudal part of the persistent 

 portion of the right subcardinal vein (V.scard.d.) described below. 

 This channel from its relation to the ureter and right Wolffian 

 body, corresponds perfectly with Hochstetter's 'Urnierenvene.' 

 It is of importance as Hochstetter ('93) pointed out, in that from * 

 its cephalic portion and the tributary mesonephric sinusoids 

 there probably arises the right spermatic vein. 



The channel extending from the vena sacralis media, dorso- 

 medial to the ureter (supra-ureteral), is much larger than the 

 sub-ureteral. It extends cephalad, lateral to the aorta, and 

 receives as tributaries the last two thoracic and first four lumbar 



