VEINS AND LYMPHATICS IN TRAGULUS 197 



a tendency to reduplicate its channel; the redundant element 

 then lies mesial to it. In the region of the promontory several 

 small dorsal tributaries enter the vessel. 



4- The pre-cardinal veins (fig. 2, 2). These vessels consti- 

 tute the fourth pair of symmetrical channels and appear in the 

 typical position of the early stages. The main trunk is parallel 

 to the neural tube and descends toward the Cuvierian duct, 

 into which it enters, after curving sharply forward and inward. 

 It follows the direction of the neural axis cephalad, adapts itself 

 to the cervical flexure, and turns forward nearly at right angles 

 to its descending portion. In this way the vessel may be 

 regarded as presenting two limbs, a, an arched or horizontal limb 

 which is assuming the position of the vena capitis lateralis, and 

 b, a straight or descending limb in the jugular line. The straight 

 limb receives several small, dorsal tributaries whose develop- 

 ment plays an important role in the genesis of the veno-lym- 

 phatics. 



5. The perimesonephroic plexus (fig. 2, 8). This forms a 

 dense network around the mesonephros throughout its entire 

 length. It has its greatest density along the ventral surface of 

 the organ. At many points the large mesonephroic sinuses com- 

 municate with the plexiform channels. The plexus becomes much 

 reduced in size on the mesial and lateral surfaces of the meso- 

 nephros, where an irregular series of anastomosing channels 

 establish communication with the post-cardinal vein. During 

 the course of development, the ventral portion of the peri- 

 mesonephroic plexus takes part in the evolution of certain 

 axial vessels; the mesial anastomosing branches give rise to 

 the lateral portion of periaortic plexus (fig. 3, 1+6 and fig. 4, 

 46). 



6. The umbilco-post cardinal plexus fig. 2, 7) . This connects 

 the umbilical and post-cardinal veins. It represents the inter- 

 mediate phase during which the umbilical vein has preserved 

 its capacity as a drainage line for the body wall and is about to 

 give this function over to the post-cardinal. In many places 

 the plexus has already broken down and appears to be selecting 

 the post-cardinal as its ultimate channel. 



