206 George 8S. Huntington and Charles F. W. McClure. 
precardinal tributaries. The process of fenestration not only 
involves the portions of the precardinal vein intermediate between 
the terminals of its dorsal somatic branches (1, 2, 3 and 4, in fig. 8.), 
but also invades the terminals themselves (fig. 10) which usually 
appear enlarged and dilated in a funnel-shaped fashion at their 
points of union with the main trunk. 
With the more marked separation of the veno-lymphatic plex- 
uses from the main venous channels the interfenestral spaces 
become drawn out into elongated channels which function as num- 
erous secondary avenues of communication between the anlages 
of the jugular sac and the systemic veins. 
The process of secondary capillary perivenous formation lead- 
ing to ultimate fenestration may first involve the main precardinal 
vein and then extend into the dorsal tributaries of the vein, or it 
may begin in the funnel shaped terminals of the tributaries them- 
selves, and subsequently progress in them to such an extent that 
the original single dorsal tributary becomes split up, or separated 
into two or more components, depending upon the type of the 
plexiform and fenestral development (fig. 11). 
This separation of the dorsal tributaries of the precardinal vein 
into a number of components is a fundamental process of veno- 
lymphatic development, since certain of the elements, thus second- 
arily derived, constitute, in part, the veno-lymphatic anlages of 
the jugular lymph sac, while others retain the original character 
of the early dorsal somatic tributaries, prior to veno-lymphatic 
organization, and form the series of dorsal precardinal branches, 
for the most part temporary, which lie in line with those opening 
further caudad into the promontory and the postcardinal vein. 
Prior to the detailed consideration of the development of the 
jugular lymph saes it is advisable to determine definitely the 
status of the dorsal precardinal tributaries and their share in the 
development of the veno-lymphatic anlages of the jugular lymph 
sacs. 
Although the general principles underlying the development 
of the jugular lymph sacs are the same for all embryos, the range 
of individual variation is considerable. No two embryos of 
approximately the same measurement, in the series examined by 
