Development of the Jugular Lymph Saes. 291 
mm. embryo). In other words, the jugular and subclavian ap- 
proaches retain exactly the same relations to the promontory, the 
cephalic vein and the thyro-cervical artery in the present case 
as they do in the 11 mm. embryo (figs. 51 and 52) where they com- 
municate with the systemic veins. 
The significance of the jugular and subclavian approaches, as 
well as their relation to the early veno-lymphatic plexus, should be 
constantly kept in mind in reading the following pages, since it is 
through them that the permanent connections between the lym- 
phatic and venous systems are subsequently established. 
3 Anteriorly, the veno-lymphatie sac opens by a broad ori- 
fice into the straight segment of the precardinal near the level of 
the cephalic arch, and is also connected with the same by asmaller 
channel slightly caudal to this point. This wide-open communica- 
tion between the veno-lymphatic sac and the precardinal vein consti- 
tutes the anterior tap of evacuation which on the left side of this 
embryo is much more extensive in character than in the 10 and 11 
mm. embryos (figs. 50, 51 and 52. This extensive entrance into 
the vein, which now serves as an outlet for the blood contents of 
the sac, is connected with the area of the precardinal which for- 
merly received the dorsal tributaries 1, 2 and 3, and its mode of 
origin in this case was probably the same as that of a similarly 
situated element which was described in connection with series 
19 (9 mm., fig. 42). 
The genesis of this anterior tap of evacuation can be further 
traced from the preceding 10 and 11 mm. stages, by comparing 
figs. 47, 48, 49, 50, 51 and 52 with fig. 57. <A comparison of 
figs. 47, 48 and 49 with fig. 57 will show that the area of the tap 
of evacuation in the latter corresponds to the area of the veno- 
lymphatic derivatives of tributaries 1, 2 and 3, as seen in the for- 
mer, before confluence of the individual elements to form the con- 
plete sac has occured. 
In fig 50 (series 101, 10 mm.,right side) the anterior tap of 
evacuation is small but distinct,the dorsal plexus arching to its 
precardinal junction over the combined trunks of the first and 
second spinal nerves. Cephalad of this point the junction of the 
arched and straight segment of the precardinal carries a dilated 
