Development of the Jugular Lymph Sacs. 219 
Figs. 12 and 13 indicate diagrammatically the origin of the veno- 
lymphatic components of the jugular lymph sac above described 
in detail, and their partial fusion to form the single jugular sac of 
the later stages. 
In the succeeding stage (figs. 14 and 15) the veno-lymphatic 
plexus further condenses by amalgamation and fusion of the 
primary dorsal and ventral divisions, and the communications 
with the main venous channels are reduced typically to three: 
(1) Tap A (figs. 14 and 15). The Anterior Tap of Evacuation. 
(2) Tap B (figs. 14and15). At the Jugulo-promontorial Angle, 
the future common jugular confluence, i. e., junction of internal 
jugular vein (precardinal) with common trunk formed by union 
of external jugular and cephalic veins. 
(3) Tap C (figs 14 and 15). At the site of the connection of 
the primitive Ulnar Vein with the promontory, corresponding, 
approximately, to the future jugulo-subclavian junction. 
The veno-lymphatic plexus as a whole is filled with blood and 
the plexiform character of the capillary network from which it 
arose is still manifested by the multiplicity of the fenestral spaces. 
On transverse section the walls of the original vessels entering into 
its composition still appear as partitions or septa which divide the 
interior of the sac into a complicated system of interecommunicat- 
ing channels. Later, these septa become greatly reduced in num- 
ber and extent so that there results a capacious sac lined smoothly 
by an endothelial layer, continuous with the intima of the veins. 
This sac subsequently becomes emptied, the contents being 
evacuated through the connections still persisting with the general 
venous system (fig. 15, Taps A, Band C). This process of evacu- 
ation occurs with great rapidity, so that in embryos of the appro- 
priate stages (10.5 to 12 mm.), it frequently happens that the sac 
on one side is completely emptied of its blood-contents, while on 
the opposite side it is still well filled with blood. The evacuation 
occurs normally at the above mentioned points, where the veno- 
lymphatic sac retains longest its connection with the venous sys- 
tem. On account of the small openings by means of which the 
veno-lymphatie sac usually communicates with the veins at Taps 
B and C, we have reason for believing that, in the majority of 
