480 A. M. MILLER 



(3), there is a venous plexus that receives dorsal branches cor- 

 responding to the segmental nerves (9). This plexus shows the 

 incipient stages of the formation of a longitudinal trunk, and in 

 a few places sends anastomotic branches across the medial line 

 to a corresponding plexus on the opposite side. At the level of 

 the duct of Cuvier (S), and lateral to the aorta, there is a very 

 distinct plexus of veins which receives intersegmental branches 

 from the region of the vertebral anlagen, and which converges 

 ventrally to form a discrete and fairly large vessel. This vessel 

 opens into the dorsal side of the cardinal directly opposite the 

 duct of Cuvier (5). The more cephalic venous channels are 

 confluent with those situated at the level of the Cuvierian duct, 

 so that if a longitudinal trunk is formed out of the former it will 

 obviously join the above mentioned vessel that leads into the 

 cardinal, and will constitute the longitudinal vertebral vein 

 (4a) . At the level of the brachial plexus of nerves (9a) , the dor- 

 sal somatic veins (4) open separately into the postcardinal (2) 

 which is now broken up into a number of smaller channels in 

 the mesonephros. 



The lateral group of vascular islands and channels (5) is dis- 

 tinguished from that of the preceding stage by two main features. 

 In the first place many of the islands have coalesced to form chan- 

 nels of considerable length. In the region of the precardinal these 

 channels show a decidedly segmental arrangement. At the level 

 of the duct of Cuvier (5) and thence for a short distance caudad, 

 they are characterized by a more irregular plexiform arrange- 

 ment. In the second place the vascular channels in question 

 communicate more freely with the cardinal veins (1, 2). There 

 is one very distinct vessel, representing a convergence of the more 

 cephalic channels, that opens into the lateral side of the precar- 

 dinal (1). Another vessel, which arises through the convergence 

 of a number of the channels dorsal to the duct of Cuvier, opens 

 into the trunk formed by the converging dorsal somatic plexus. 



The majority of the segmentally arranged vessels of the lateral 

 group (5) receive dorsal branches from the mesenchymal tissue 

 between the vertebral anlagen and the epidermis. At the level 

 of the duct of Cuvier (5), two or three of these prolongations have 



