250 George 8. Huntington and Charles F. W. McClure. 
veno-lymphatic element, (4VL, fig.37) appears asan extensive and 
fenestrated sac. It ends anteriorly in a blind diverticular ex- 
tremity. Two channels of communication through X”” and X’ 
lead into the main precardinal channel, between which a trans- 
verse branch connects a medial somatic tributary (48’) with the 
veno-lymphatie component of tributary 4 (4LV), Proceeding 
eaudad, the veno-lymphatic sac of 4 enlarges and presents 
multiple fenestral formation in line with the fenestre of the pro- 
montorial and primitive ulnar areas, and unites with the redun- 
dant veno-lymphatie dilatation of this region (dorsal veno-lym- 
phatic plexus). Medially it connects by six transverse branches 
(a,b,c, d,e,f) with a well developed dorso-medial trunk (4S) which, 
as the main representative of the somatic element of 4, enters the 
dorso-medial angle of the promontory in line with the succeeding 
dorsal somatic promontorial branches 5S and 68, ete. 
It will be noted that in the general plan of organization the two 
sides of this embryo agree closely and present a consistent picture, 
characterized by three main features. 
1. Distinct development of a dorso-lateral series of dilated 
veno-lymphatic elements derived from the precardinal tributaries 
1, 2and 3 and from precardinal tributary 4 and its associated para- 
precardinal channel. Of these the derivatives of tributaries 1, 2 
and 3 represent the anlage of the cephalic division of the ventral . 
veno-lymphatic plexus, while the element derived from the tribu- 
tary 4 and the para-precardinal channel forms the anlage of the 
caudal division of the ventral veno-lymphatic plexus. In early 
stages the latter is often continuous with the dorsal veno-lym- 
phatic plexus, which is derived from the dilated, plexiform dorso- 
lateral aspect of the promontory and the primitive ulnar segment 
of the postcardinal. 
2. Coincident with this development is the appearance of 
a series of dorso-medial branches which represent the somatic 
elements of the primitive precardinal tributaries. These are very 
variable in their character, as may be seen by comparing the oppo- 
site sides of the embryo (figs. 34 and 36). 
3. The most remarkable feature is the persistence between the 
dorso-lateral veno-lymphatic and dorso-medial somatic elements 
of a series of transverse communications. 
