Development of the Jugular Lymph Saes. 295 
secondary connections, it is to be expected that certain embryos 
will be fixed at periods in which, on the one hand, the temporary 
detachment has been nearly, but not quite completely attained, 
and, on the other, in which the secondary connections are just be- 
ginning to be established. In either case the resulting histolog- 
ical picture will yield these so-called ‘‘doubtful’’ connections. 
The evidence that the jugular lymph sac becomes temporarily 
separated from the veins is based upon the following observations: 
1. We have been unable to find any connection between sac 
and vein on the right side of embryo 78 (12 mm.). 
2. There is no connection on the left side of this embryo 
series 78 at either of the typical points of final lymphatico-venous 
entry, viz., the common jugular confluence and the jugulo-sub- 
clavian angle. 
3. No connection between sac and veins exists at any point 
in all mm. embryo (series 27), in which the jugular lymph sac 
is fully established. 
4. No connection is found in the 13 mm. embryos. 
5. Absence of the typical communications in the Harvard 
series 474 (10.7 mm), although the minute doubtful connections 
above described may be present. 
The definite and permanent connections of the jugular lymph 
sac and the veins do not appear prior to the stage between 14 and 
15 mm., from which stage on they are usually constant. 
Reconstruction of left side, lateral aspect (slides C, D, E, and F) fig. 58. 
The jugular lymph sae contains no blood and is composed of 
clearly defined confluent elements, (fig. 58), as follows: 
1. The part derived from the cephalic division of the ventral 
veno-lymphatic plexus (J). 
2. The part derived from the caudal division of the ventral 
veno-lymphatie plexus (//) terminating in a blind prolongation 
(jugular approach), which is directed toward, and nearly reaches, 
the angle of confluence of the internal jugular and the combined 
trunk of the external jugular and cephalic veins (common jugular 
angle). 
