Development of the Jugular Lymph Saes. 195 
Ina 15mm. embryo (series 75) the caudal end of the left jugular 
lymph sae (subclavian approach) was filled with blood corpuscles 
and communicated with the venous system in an unmistakable 
manner at the jugulo-subclavian junction, by means of a wide 
opening (slide 8, section 22, left side). A doubtful communication 
was also apparent at a corresponding point on the right side, but 
in neither case could one be detected at the junction of the external 
and internal jugular veins. In another 15 mm. embryo (series 53, 
slide 11, sections 23 and 24) the caudal end of the lymph sac was 
in wide-open communication with the vein on both sides of the 
embryo, at a point slightly in front of the jugulo-subclavian June- 
tion. These connections in the 15 mm. embryo may be secondary 
taps, fully established, but not yet invaginated into the typical 
wedge-shaped entrance, and prior to the valve formation. 
In a 15.5 mm. embryo (series 141) no indication of a communi- 
eation could be observed on either side. In this case either the 
measurement of the embryo may have been modified by varia- 
tions in curvature, or the secondary connections belated. 
From the 16 mm. embryo on, in which the thoracic duct. usually 
forms a continuous vessel and communicates with the left jugular 
lymph sac, we have found no difficulty in determining the pres- 
ence of a lymphatico-venous connection or tap at either one of the 
two, or at both of the points at which the lymphatic system com- 
municates with the veins in the adult. The lymphatico-venous 
connection or tap met with in these more advanced embryos 
differs widely in character from that found in the 15 mm. embryos 
as well as from that found in the preceding veno-lymphatic stages, 
in which a wide-open communication may be present. The lym- 
phatico-venous connection in these advanced embryos is formed 
at each of the two typical adult points of entry by a wedge-shaped 
process of the jugular lymph sae which is deeply invaginated into 
the angle of confluence of two veins, or into one of the veins econ- 
tiguous to the point of confluence, and which opens into the vascular 
lumen by a narrow slit-like aperture bounded by a two-lipped 
valve. This type of embryonic lymphatico-venous connection 
was first demonstrated by the writers at the meeting of the 
