Development of the Jugular Lymph Sacs. 211 
though not invariably, happens that a portion of the same becomes 
partially or wholly detached and is displaced to the lateral sur- 
face of the precardinal vein and the promontory. 
This feature of development appears to be of sufficient importance 
to warrant a careful consideration, as it further emphasizes the 
~variable character of the process of fenestration, as well as the 
variable form which may be assumed by this division of the ven- 
tral plexus as the result of this process. 
The two embryos in question illustrate exceedingly well the 
partial separation of a portion of the ventral plexus and its dis- 
placement to the lateral surface of the promontory and precardi- 
nal vein. They also give us the clue as to the origin of the inde- 
pendent and completely isolated veno-lymphatic sacs or spaces 
which are met with in certain cases on the lateral surface of the 
promontory and precardinal vein (fig. 50). 
Series 112, 10°” Embryo 
‘Reconstruction of left side, 
Lateral aspect, fig. 47 
The Caudal Division of the Ventral Veno-Lymphatic Plexus. 
The caudal division of the ventral vcno-lymphatic plexus in 
this embryo (fig. 47) is a less condensec and compact structure 
than in the 8.5 mm. embryo (figs. 44 and 46), Just considered. 
This does not necessarily mean that it is less advanced in develop- 
ment than in the 8.5 mm. embryo (series 102), but rather that 
the plexiform character has been longer retained in it than in the 
case of the latter, at a stage which precedes the general amalgama- 
tion of all the veno-lymphatics to form a single common sac. 
In this embryo (fig. 47) the caudal division of the ventral veno- 
lymphatic plexus consist of three distinct and parallel channels. 
A large dorsal channel, continuous with the dorsal veno-lvm- 
phatic plexus, which joins anteriorly the ventral of the three 
channels by two transverse vessels; an intermediate smaller 
channel which opens into the lateral surface of the promontory 
at Tap B’ and into the dorsal surface of the ventral channel, and 
of a large ventral channel lying upon the lateral surface of the pre- 
