276 ~=George 8S. Huntington and Charles F. W. McClure. 
2. Posterior Area of the caudal division of the ventral plexus. 
In this embryo, as clearly indicated by the course of the thyro- 
cervical artery, the caudal division of the ventral plexus receives 
an addition of plexiform veno-lymphatiec anlages (fig. 48, #’, H”, 
E’"’) which communicate with the lateral surface of the promon- 
tory by five openings. This veno-lymphatic plexus intervenes 
between the caudal termination of the two elements (V and D) 
forming the anterior portion of the caudal division of the ventral 
plexus, and the dorsal veno-lymphatic plexus. They are undoubt- 
edly destined to fuse with each other and with the remaining 
components of the caudal division. After this fusion has occurred 
the thyro-cervical artery will then occupy its typical and invari- 
able position, arching over the shoulder of the promontory be- 
tween the dorsal veno-lymphatie plexus and the united caudal 
division of the ventral plexus. 
The Dorsal Veno-Lymphatic Plexus. 
The dorsal veno-lymphatie plexus of this embryo (fig. 48) is 
still incompletely separated from the promontory. It forms a sac- 
like fenestrated structure which opens into the distal portion of 
the promontory (fig. 48, Tap C), as well as into the primitive ulnar 
arch. 
The Cephalic Division of the Ventral Veno-Lymphatic Plexus. 
The veno-lymphatic components of precardinal tributaries 1, 
2 and 3 (fig. 48, 1VL, 2VL 3VL), which will unite to form the 
cephalic division of the ventral plexus, are still individually 
distinct; IVL and 2VL are united by a narrow channel, 3VL is 
still separate, partially hidden in the figure by the second spinal 
nerve (fig. 48, SP.N.IT). 
In addition to the separation of the caudal division of the ven- 
tral plexus into separate components and the displacement of 
one of these components from the dorsal to the lateral surface of 
the promontory, this embryo is characterized by the large number 
of discrete connections which still persist between the veno-lym- 
phatics and the systemic veins. Beginning with precardinal tribu- 
