Development of the Jugular Lymph Saes. 289 
with blood and remain in free communication with the veins. Be- 
tween this veno-lymphatic stage and the stage of final organiza- 
tion of the lymph saes as definite portions of the entire lymphatic 
system, intervenes a very short period which we have designated 
as the pre-lymphatic stage. In this stage the venous derivatives 
appear fused together to form a single sac which then undergoes 
the following successive changes; 
1 The blood contents of the sac are emptied in many cases 
into the systemic vein through a large connection, secondarily 
established, between the anterior end of the common sac and the 
precardinal vein (Anterior tap of evacuation). Except for this 
anterior connection between the systemic veins and the sac, the 
latter is distinctly lymphatic in character, and, from a histological 
standpoint, cannot be distinguished from the lymph sac of much 
later stages. It is possible that in certain embryos the promon- 
torial connections serve as posterior taps of evacuation, in which 
ease the anterior tap is not established. 
2 After the sac has become emptied of blood contents the an- 
terior tap of evacuation closes rapidly, and the sac, according to 
our observation, separates for a short time entirely from the ven- 
ous system. <A few doubtful points of apparent communication 
observed in some instances are very likely artifacts. In any case 
they differ entirely in character and appearance both from the 
earlier connections of the veno-lymphatic plexus with the veins, 
and from the later typical lymphatico-venous taps found after 
the adult conditions have been established. 
Although in this pre-lymphatic stage no definite promontorial 
connection between the sac and the veins can be determined, 
the sac sends out two processes which approach and almost reach 
the two points which in the adult constitute the area of lymphatico- 
venous connections. These two points correspond, respectively, 
to those at which in the earlier stages the dorsal and ventral veno- 
lymphatic plexuses opened into the jugular promontory. (Com- 
pare fig. 16 with figs. 14, 15 and 17). It is difficult to account on 
physiological grounds for this apparent temporary separation of 
the sac from the veins. Since the permanent connections of the 
lymphatic and venous system take place apparently at the iden- 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 10, NO. 2. 
