Development of the Jugular Lymph Saes. 199 
These additional tributaries (b’, b”, b’’’) caudad of the main 
trunk of A-B are serially more or less in direct line with the set 
of primary dorsal precardinal tributaries (1, 2, 3 and 4 in fig. 8) 
entering into the subsequent formation of the veno-lymphatic 
plexuses. As far as our observations extend, tributaries b’, b’’ 
and b’” do not generally enter into the composition of these 
plexuses, although it is possible that in the later stages, in cer- 
tain circumstances, they may be retained as secondary anterior 
channels of communication between the fully established veno- 
lymphatic sac and the precardinal vein. 
The possible involvement of these tributaries (b’, ete.) of the 
cephalic arch in the late veno-lymphatie stages, in connection 
with the establishment of an anterior tap of evacuation is discussed 
in dealing with the series concerned (ef. p. 292, series 78, fig. 57), 
B. Ventral Tributaries of the Cephalic Arch. 
In the early stages the concavity of the cephalic arch receives 
a number of tributaries which temporarily drain a territory sub- 
sequently drained by the permanent external jugular vein. The 
vagus and spinal accessory nerves lie in close relation to these 
branches, and in later stages these vessels anastomose around the 
spinal accessory so that the nerve then penetrates the precardinal 
arch from the medial to the lateral aspect through an oblique 
foramen. 
2. Caudal or Straight Segment of the Precardinal Vein. 
The cephalic arch is continued caudad into a second nearly 
straight segment of fairly uniform caliber. This portion of the 
vein is the more important of the two, since both the dorsal cir- 
cumference of the vein itself and its dorsal tributaries (1, 2, 3 and 
4, in fig. 8) are involvéd in the subsequent development of the 
veno-lymphatiec plexuses. 
A. Dorsal Tributaries of the Caudal or Straight Segment of the 
Precardinal Vein. 
This segment may receive four distinct and separate tributaries 
(1, 2, 3 and 4, fig. 8) or four distinct groups of tributaries, depend- 
