Development of the Jugular Lymph Saes. 303 
to the subclavian, thyro-cervical arteries and to the sympathetic 
nerve remain the same as described above for the 16 mm. embryo. 
Two small lymphatic vessels lying along the internal jugular 
vein open into the medial aspect of the jugular sae just in front of 
the entrance into the latter of the thoracic duct. These vessels 
represent the approach of the sac to union with the lymphatic 
vessels which have begun to form along the course of the internal 
jugular vein. 
The fifth spinal nerve (SP.N.V.) still penetrates the jugular 
sae and the relations of the thyro-cervical artery and its branches 
to the jugular lymph sac and systemic veins are the same as in 
the 16 mm. embryo (fig. 62). 
Series 88, 18" Embryo 
Reconstruction of the jugular lymph sac of the right and left sides 
Ventral View, fig. 64 and 
Lateral view of the left side, fig. 66 
This reconstruction is especially valuable in showing the sym- 
metrical degree of development obtained by the jugular lymph 
sac on opposite sides of the same embryo, as well as the fact that 
up to acertain stage of development both sacs are ontogenetically 
of equal importance (fig. 64). 
In the 18 mm. embryo the arteries end main systemic veins 
have practically reached the adult concition. The external and 
internal jugulars are more elongated than in the preceding stage 
as the result of the continued descent of the heart. The external 
jugular is a vein of large size, although its caliber does not yet exceed 
that of the internal jugular. The cephalic vein is also of large 
size and,after joining the external jugular to form the common 
jugulo-cephalic trunk, the latter, which has become somewhat 
elongated, joins the internal jugular at the common jugular angle. 
The resulting common trunk formed by the confluence of the 
internal. jugular and jugulo-cephalic trunk, as far caudad as its 
confluence with the subclavian, now constitutes the common 
jugular vein (see fig. 1 of adult). 
Beginning in the 17 mm. embryo, slightly caudal to the level 
of the subclavian vein and dorsal to the thymus glands, the two 
