Development of the Jugular Lymph Saces. 305 
along the lateral surface of the jugular sac in earlier stages (fig. 62) 
is embedded in the lymph sac in this 18 mm. embryo (fig. 64) 
with an accompanying branch of the cephalic vein. 
Lymphatic formation, independent of the jugular lymph sae, 
is in full swing,and some of these independent lymphatics have 
made secondary connections with the jugular sacs. Closed lym- 
phatic channels or sacs stud the course of the jugular, cephalic 
and subclavian veins, and an especially prominent lymphatic 
follows the course of the diminishing left precava (fig. 65). 
Series 22, 25°" Embryo 
Reconstruction of left side, 
Ventral aspect, fig. 66 
Only the caudal end of the lymph sac and its related veins are 
shown in the figure. 
As far as the jugular lymph sac is concerned, the final approach 
to the adult condition is achieved in the 25 mm. embryo. There 
is a further reduction in the size of the internal and a correspond- 
ing enlargement of the external jugular vein. The common vessel 
(jugulo-cephalic trunk) formed by the confluence of the external 
jugular and cephalic veins has, with further cardiac descent, be- 
come much elongated and the common jugular angle lies rela- 
tively further caudad than in the 18 mm. embryo. This descent 
has drawn out the lymph sac of the preceding stage into an elon- 
gated channel which lies along the veins and taps them in this 
case also at the two typical points. 
The dorsal and ventral divisions of the subclavian approach, 
described above in connection with the 17 mm. embryo (fig. 63), 
now anastomose with each other just caudal to the jugulo-sub- 
clavian confluence, where they become continuous with a lymph 
channel which follows the left precava. This latter lymphatic 
was described in connection with the 18 mm. embryo (left pre- 
vacal lymphatic, fig. 65), where a connection between it and the 
jugular sac does not yet exist. 
Numerous independent lymphatic structures are found along 
the course of the large veins. Among these may be mentioned a 
large complex lymphatic sac which envelops the subclavian vein, 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 10, No. 2. 
