Development of the Jugular Lymph Sacs. 197 
then appears to become temporarily detached from the venous 
channels from which it arose. 
4. SECONDARY OR DEFINITE LYMPHATIC STAGE, In which the evac- 
uated sac becomes assigned to the lymphatic system, establishing, 
on the one hand, connections with the independently developed 
general systemic lymphatics, and, on the other, reéntering the 
permanent venous channels by apparently secondary connections 
made at typical points, thus forming the link which in the adult 
unites the definitely organized venous and lymphatic systems. 
It is desirable, for reasons which will appear in the presentation 
of the subject, to deal with the individual problems involved on the 
following basis and in the order indicated: 
I. General Ground-Plan of the Embryonic Venous Area 
Involved in the Subsequent Development of the Jugular 
Lymph Sacs. 
II. Analysis of the Developmental Stages in the Formation 
of the Jugular Lymph Sacs. 
III. Detailed Description of the Individual Stages. 
IV. Summary and Conclusions. 
I. GENERAL GROUND-PLAN OF THE EMBRYONIC VENOUS AREA 
INVOLVED IN THE SUBSEQUENT DEVELOPMENT OF THE JUG- 
ULAR LYMPH SACS. 
It is necessary to consider in the first place the general struc- 
tural ground-plan of the principal embryonic venous channels 
concerned in this process, and to establish certain main divisions 
of the same to which subsequent reference can be made in dealing 
with the individual series. 
The portion of the embryonic venous system involved in the 
development of the jugular lymph sacs includes a large part of 
the precardinal vein, the district of its confluence with the post- 
cardinal vein to form the duct of Cuvier, and the proximal seg- 
ment of the postcardinal vein caudal to this point. 
This venous area can be mapped out into the districts shown in 
the composite average schematic fig. 8 containing the following 
subdivisions: 
