Development of the Jugular Lymph Sacs. 203 
main channel of the pre- and posteardinal veins, adjacent to 
and including their point of confluence to form the duct of Cuvier. 
Two distinct ontogenetic types are involved in the production of 
this structure: 
1. A certain number of dorsal somatic tributaries of the early 
precardinal vein become dilated and fuse together, retaining at 
first their primary connections with the main vein. This process 
in the mammalian embryo, preliminary to definite lymphatic 
formation, is strictly homologous to the development of the lymph 
hearts in the lower vertebrates. Sala’s'! researches on the 
development of the posterior lymph heart in the embryo of the 
common fowl prove that this organ is developed entirely by dilata- 
tion and fusion of the lateral branches of the first five coccygeal 
veins. 
In the mammalian embryo, and specifically in embryos of Felis 
domestica, the first four dorsal somatic branches of the straight 
or caudal segment of the precardinal vein are, to a greater or less 
extent, involved in the formation of the jugular lymph sac, the 
homologue of one of the lymph hearts of lower vertebrates. 
2. Thus, while in mammals this phylogenetic type of lymph 
heart formation is preserved, it plays but a secondary part in the 
production of the jugular lymph sac. The greater part of this 
structure, which in certain embryonic stages reaches a relatively 
enormous development; as compared with its condition in the 
adult, is directly derived from the main systemic veins by a proc- 
ess, which we have called ‘“‘fenestration.”” By this designation we 
do not mean to define a method of histogenetic formation of the 
early embryonic blood vessels, but to accentuate, by the use of a 
brief and serviceable term, the fact that in certain stages in the 
development of the venous system, stages of the greatest impor- 
tance in preparation for the appearance of the veno-lymphatic or 
lymph heart division of the lymphatic system, the early embry- 
16 Sala, Luigi, Ricerche fatte nel laboratorio de Anatomia Normale della R. 
Universita di Roma: vol. 7, pp. 263-269, April, 1900. 
