Development of the Jugular Lymph Sacs. 213 
complete the secondary dorsal veno-lymphatie arch of later stages 
(fig. 51, series 77). 
The primitive ulnar veno-lymphatic has become detached from 
the vein from which it arose by condensation of the surrounding 
capillary network, and now enters the caudal extremity of the 
dorsal veno-lymphatiec plexus. 
The relations of the thyro-cervical artery to the dorsal veno- 
lymphatic plexus and to its ventro-caudal extension, the sub- 
clavian approach, are the same as in the preceding embryo (series 
102, fig.44). The jugular approach is represented by that portion 
of the ventral plexus which opens into the promontory at the two 
points labelled Tap B and Tap B’ in fig. 47. 
The Cephalic Division of the Ventral Veno-Lymphatic Plexus. 
The veno-lymphatice components of tributaries 1, 2 and 3 which 
constitute the anlages of this division of the ventral plexus (1, 2 
and 3VL, fig. 47), although modified by retiform fenestration, still 
retain their individuality in this embryo, a condition which may 
or may not obtain up to a late stage of development. 
The embryo offers a good concrete example of the following 
general phases of veno-lymphatic development: 
1. Unequal development of the two main veno-lymphatic 
plexuses at any given stage. 
2. Beginning separation of the veno-lymphatic sac, formed by 
further condensation of the plexuses from the definite venous 
channels. 
3. The formation of isolated and completely closed sacs or 
spaces, detached both from the veins and from the main veno- 
lymphatic plexuses, which are subsequently incorporated in the 
jugular lymph sae. 
4. The genesis of the secondary dorsal veno-lymphatic bridge 
or arch, which in certain subsequent stages connects the cephalic 
end of the dorsal plexus with the anterior veno-lymphatic channels 
of the ventral division. This structure which, e. g., appears in 
its characteristic development in series 77 and 101 (figs. 51 and 50), 
is apparently developed in different individuals to an unequal 
degree and in one of the following ways: 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 10, NO. 2. 
