282 George S. Huntington and Charles F. W. McClure. 
The remaining portion of this plexus presents an appearance 
which has been shown to be typical for this structure in a number 
of the preceding stages (e. g. fig. 48). It consists of a vessel running 
parallel to the precardinal, somewhat arched anteriorily, which 
opens into the precardinal at two points (Tap X and Tap X’, 
fig. 50) and into the lateral surface of the promontory by a single 
orifice (Tap B). Opposite its point of communication with the 
precardinal at Tap X’, it is connected by means of a very narrow 
stalk with a dilated oval blind sae whose caudal extremity ap- 
proaches the promontory just in front of the thyro-cervical artery. 
This expanded appendage to the caudal portion of the ventral veno- 
lymphatic plexus is an important feature of development and 
appears to represent a condition frequently obtained by this plexus 
previous to its subsequent amalgamation with the dorsal plexus 
and the cephalic division of the ventral veno-lymphatic plexus to 
form a common sac (cf. fig. 14). 
Although exactly the same conditions seldom prevail in any 
two embryos, it is not difficult to interpret the origin of the caudal 
division of the ventral plexus in the present instance on the basis 
of a further development of the conditions shown in series 19 
(fig. 41, 9 mm. embryo). 
The channel of the caudal division of the ventral veno-lymphatic 
plexus which, in the 10 mm. embryo (fig. 50, series 101) runs 
parallel to the precardinal and taps the systemic veins at three 
points (Tap X, X’ and B), has its homologue in the 9 mm. embryo 
(fig. 41, series 19) in the para-precardinal channel and the element 
L which has been separated from the latter, while the oval blind 
sac in series 101 (fig. 50) which is connected with this parallel 
channel by a narrow stalk has been derived from the veno-lym- 
phatiec component of precardinal tributary 4 in series 19 (4VL, 
fig. 41). On the other hand, the two detached and closed sacs in 
series 101 (fig. 50) are the secondary products of the original para- 
precardinal plexus (fig. 41, see also fig. 31) which have become 
separated from the same by an increase in the process of conden- 
sation, as in figs. 47 and 48. 
The transformation of the complex of tributary 4 and the para- 
precardinal channel into two or more parallel channels is a feature 
ee eee 
