256 George S. Huntington and Charles F. W. McClure. 
plexus of the dorsal precardinal tributaries of earlier stages as the 
result of the condensation into dorsal somatic and veno-lymphatic 
components (see description of series 138, and figs. 33 and 35). 
The veno-lymphatiec component of tributary 1 (1VL, fig. 40) 
is dilated dorsally into an elongated sac which opens into the dorso- 
lateral circumference of the precardinal by a slender channel. 
From its general appearance and relations to the area of the pre- 
cardinal occupied by precardinal tributary 2, it is possible that this 
dilated sac may be a compound structure formed through a fusion 
of veno-lymphatics derived from tributaries 1 and 2. After 
fusing with the more caudally situated veno-lymphatics, however, 
‘it will form the most anterior portion of the continous dorsal 
veno-lymphatic sac of later stages and its primary connection 
with the precardinal vein may serve as the channel of exit into the 
systemic veins of the blood contained in the veno-lymphatic 
sac (see figs. 12, 13, 14 and 15). 
The area of the precardinal occupied by tributary 2 (2 VL) is 
redundant and fenestrated, a condition which foreshadows the 
formation of an anterior para-precardinal channel which may 
involve in its separation from the precardinal the bases of trib- 
utaries 1, 2 and 3, At a later stage this channel frequently 
fuses along the lateral surface of the precardinal with an anterior 
prolongation of the caudal division of the ventral veno-lymphatic 
plexus (see figs. 12, 18, 14 and 15). 
The veno-lymphatic component of precardinal tributary 3 
(3VL) is, relatively, a large and somewhat dilated, single vessel 
which opens dorso-laterally into the precardinal, caudal to the 
territory occupied by tributary 2 (2VL). 
Precardinal tributary 4 presents an interesting condition in 
this embryo and one which, in some respects, resembles that 
found on the right side of the 7.25 mm. embryo (fig. 39), since it 
has severed its connection with the para-precardinal channel 
and the precardinal vein. The primary point of connection which 
formerly existed between tributary 4 and the precardinal vein 
(X, fig. 40) is indicated in the figure by broken lines. 
In the area of precardinal tributary 4 and its associated para- 
precardinal channel the process of fenestration is completed. A 
