Development of the Jugular Lymph Sacs. 293 
The dorsal and ventral portions of the sac are clearly differen- 
tiated on the left side in Fig. 55. The multilocular character of 
the empty sac and the incomplete septa and partitions in the 
interior are well shown in both figures. 
Secondary features of significance illustrated by the left side 
of this embryo are as follows: 
1. The thyro-cervical artery on reaching the level of the pro- 
montory bends laterad on to the lateral surface of the promontory 
in the interval which separates the jugular from the subclavian 
approach. On reaching the lateral surface of the promontory the 
thyro-cervical artery gives off a branch which runs dorsally along 
the lateral surface of the veno-lymphatic sac. This latter branch 
is accompanied by a branch of the cephalic vein and in later 
stages both this vein and the artery which accompanies it may 
become embedded in the jugular lymph sae. 
2. The primitive ulnar vein has given up its anterior connec- 
tion with the promontory and the drainage of the anterior limb 
is now assumed by the definite subclavian vein. The subclavian 
vein opens into the promontory slightly caudal to the point where 
the latter formerly received the primitive ulnar vein. (Compare 
fig. 51, in which the subclavian connection is established). Before 
the primitive ulnar vein gives up its anterior connection with the 
promontory, the sixth spinal nerve (SP.N.VJ) arches under or 
ventral to the primitive ulnar vein and primitive ulnar veno- 
lymphatic (figs. 45,47 and 48). With the loss of its promontorial 
connection, however, this relation is alone maintained to the 
primitive ulnar veno-lymphatie (fig. 57). Cf. figs, 13, 14, 15 and 16. 
3. The primitive ulnar veno-lymphatic forms a well-defined 
caudal prolongation of the veno-lymphatic sac and can be traced 
to the base of the anterior limb. 
4. ‘The marked convexity of the cephalic arch has been reduced 
with the descent of the heart and the resulting elongation of the 
straight segment of the precardinal. The promontory no longer 
forms a prominent swelling at the cardinal-Cuvierian junction, 
and has been reduced by the caudal descent of the heart. 
5. The relations of the spinal nerves to the veno-lymphatic 
sac have already been referred to and need no further explanation, 
