Development of the Jugular Lymph Saces. 243 
tory, and only differentiated from the same by a linear series of 
three fenestrae, represents the extensive plexiform area which in 
series 109 (fig. 27) occupies the dorso-lateral circumference of the 
promontory. The same relative reduction of the dorsal veno- 
lymphatic plexus is seen on the left side of this embryo (fig. 32) 
when compared with the same side of the 6. 2 embryo (series 109, 
fig. 27.) 
Series 138, 7°” Embryo 
Reconstruction of left side, 
Lateral aspect, fig. 33 and 
Medial aspect, fig. 34 
This embryo offers a remarkably clear picture of veno-lym- 
phatic development and illustrates well the relation of the veno- 
lymphatic plexuses to the system of the primitive dorsal somatic 
tributaries. 
The plexiform and dilated terminals of precardinal tributaries 
1, 2 and 3 (as seen in figs. 27, 29 and 31), and the terminal of 
dorsal precardinal tributary 4 and its associated para-precardinal 
channel, reveal here very strikingly their separation into dorsal 
somatic and veno-lymphatic components. Another feature of 
fundamental importance to the interpretation of succeeding 
stages is the tendency displayed by these veno-lymphatic deriva- 
tives to shift their connection with the precardinal from the dorsal 
to the dorso-lateral circumference of the main channel. 
After these veno-lymphatic derivatives have assumed their 
typical dorso-lateral position with respect to the main systemic 
venous channel, they are then in line to fuse with one another and 
with the dorsal veno-lymphatic plexus which, from the beginning, 
projects cephalad from the dorso-lateral circumference of the 
promontory. 
This embryo likewise throws considerable light on the genesis 
of the variations presented by the terminal of dorsal somatic 
tributary 4 of the early series, in reference to the double character 
assumed by the same in both the somatic and the veno-lymphatic 
direction. For these reasons the embryo requires special and 
detailed consideration. 
