110 EDWAED MCCRADY, JR. 



The paracardinal plexus. Part of the originally extensive 

 venous plexus from which the duct of Cuvier and the proximal 

 ends of the pre- and post-cardinals differentiate persists in 

 the angle between the cardinals until stage 32 when the jugular 

 lymph sac begins to form in this region. This persistent por- 

 tion is called the paracardinal plexus, and its fate will be dis- 

 cussed in detail in connection with stages 32 and 33. It is seen 

 in the reconstruction (fig. 36) to have its principal connection 

 with the precardinal, not very far from the cardinal anasto- 

 mosis. It lies immediately beneath the dermo-myotomal 

 plates, is constricted at the intersomitic levels, and communi- 

 cates by small twigs (not shown in the figure) with the seg- 

 mental veins. The first somite has disappeared, and only a 

 trace of the second is left, but the sites of both may be identi- 

 fied by the segmental vessels and by the bulges in the para- 

 cardinal plexus, which is quite sinusoidal in the somitic regions 

 at this time. 



The veins of the limb buds. It was mentioned in connec- 

 tion with the origin of the umbilical vein in stage 24 that its 

 first association is with the anterior limb bud, on the ventral 

 or radial side. Also it was pointed out that the postcardinal 

 in stage 26 established communication with the dorsal border 

 of the limb ridge. Thus when the blood begins to circulate 

 in stage 27 the rich venous plexus which ramifies through the 

 limb drains dorsally into the postcardinal and ventrally into 

 the umbilical. 



At first the plexus seems altogether irregular, but by stage 

 28 it is beginning to show a more orderly arrangement of the 

 vessels. The veins form a sort of subperipheral layer around 

 the limb bud (fig. 36, D and E), the dorsal part of this layer 

 draining by many small mouths into the postcardinal, the ven- 

 tral part in the same way into the umbilical. The arterial 

 supply from the dorsal aorta enters the medial part of the limb 

 and communicates through capillaries with all parts of the 

 subperipheral plexus. 



Caudad of the anterior limb bud both the umbilical and the 

 postcardinal veins extend all the way back to the end of the 



