208 George 8. Huntington and Charles F. W. McClure. 
II. From the Establishment of the Three Primary Veno- 
Lymphatic Plexuses or Sinuses to the Attainment of the 
Adult Condition of the Jugular Lymph Saes. 
FIRST PERIOD. 
2. FROM THE HARLY VENOUS STAGES TO THE ESTABLISHMENT 
OF THE THREE PRIMARY VENO-LYMPHATIC PLEXUSES. 
A. Development of the Ventral Veno-Lymphatic Plexuses. 
In some instances the ventral veno-lymphatic plexus is largely 
derived from the primary dorsal somatic branches 1, 2, 3 and 4, 
while in others the main portion of the plexus differentiates by 
plexiform condensation from the precardinal vein, carrying the 
tributaries with it, in which case these latter play a secondary 
part in the development of the plexus. Finally in a third group 
both somatic and main precardinal components share nearly 
equally in the formation of the plexus. Again one portion of the 
plexus may be mainly precardinal in derivation, another largely 
composed of elements contributed by the somatic tributaries. 
Hence the ventral veno-lymphatic plexus, in both of its sub- 
sidiary parts, exhibits a considerable range in variation, as will 
appear in the detailed description of the individual series, and it 
becomes necessary to analyze the two developmental processes 
in detail in order to prepare the way for the consideration of the 
conditions presented by any given case. 
Moreover, in this general plan of development of the ventral 
plexus, dorsal somatic precardinal tributary 4 and the associated 
segment of the main precardinal vein take a share approximately 
equalling that contributed by the remaining three (anterior) 
tributaries (1,2 and 3) and by the portion of the precardinal vein 
included within their area. This results in the establishment of 
the two primary divisions of the ventral veno-lymphatic plexus, 
viz: 
1. <A cephalic or anterior division. 
2. A caudal or posterior division. 
eo 
