Development of the Jugular Lymph Saes. 229 
oa ) 
There is no indication of the presence of the dorsal somatic 
element 4S, which is present on the left side of this embryo 
(fig. 22). The reconstruction suggests that in this instance, 
as frequently happens, the primitive dorsal tributary 4 becomes 
involved in its entire extent in, the development of the caudal 
division of the ventral veno-lymphatic plexus. 
Caudad of tributary 4 the elongated distal segment of the pre- 
cardinal receives dorsally three small branches (K) which may 
represent the element K of the opposite side, (fig. 22). 
Spindle shaped dilatation V/is bent sharply ventrad and receives 
two large ventral branches, which represent the anlage of the per- 
manent external jugular vein. As a matter of fact these vessels 
now open into the cardinal end of the duct of Cuvier, but with the 
caudal extension of the promontory, which is not yet fully estab- 
lished in this embryo, they will be included in later stages within 
the promontorial area. (See fig. 8, which illustrates diagramma- 
tically the progressive caudal extension of the promontory and 
the gradual inclusion within the same of the terminals of the exter- 
nal jugular vein). 
Dorsally, the postcardinal, at its confluence with the precardinal 
channel is dilated, and perforated by irregular fenestrae of varying 
size. 
Comparison with the left side (fig. 22) suggests that confluence 
of these separate openings would establish the single large fenes- 
tra there encountered, in which case the channel H! (fig. 23) 
would form the main postcardinal path, while the remaining 
dorsal fenestrated arch would yield a para-cardinal venous arch H 
which constitutes the anlage of the dorsal veno-lymphatic plexus. 
The two sides of this embryo control and supplement each 
other extremely well and present a typical picture of the stage 
concerned. 
Series 31, 5+" Embryo 
Reconstruction of left side, 
Lateral aspect, fig. 24 
The cephalic arch is somewhat more decidedly ante-flexed, and 
better differentiated against the succeeding straight segment of 
