Development of the Jugular Lymph Sacs. 235 
B. DETAILED STUDY OF THE INDIVIDUAL SERIES. 
Series 109, 6.2" Embryo 
Reconstruction of left side, 
Lateral aspect, fig. 27 and 
Medial aspect, fig. 28 
The cephalic arch is bent ina wide curve ventro-caudad. The 
tributary A-B of the convexity of the arch is of large size, and has, 
relatively, moved forward with the better differentiation of the 
arch from the straight segment of the precardinal. The area of the 
precardinal which receives the dorsal tributaries 1, 2 and 3 (in figs. 
27 and 28), is somewhat expanded and dilated along its 
dorsal circumference, a feature which characterizes this region 
of the precardinal preparatory to the separation therefrom of a 
secondary channel by crystallization of the perivenous capillary 
network. Asa result of this expansion of the precardinal vein, the 
first three dorsal tributaries of the latter (1, 2 and 3, figs. 27 and 28), 
assume a funnel-shaped form and frequently become confluent 
at their precardinal termination. This funnel-shaped enlargement 
of the terminals of dorsal tributaries 1, 2 and 3, is well shown in 
fig. 27, where tributary 1 opens into the precardinal as an inde- 
pendent vessel, while tributaries 2 and 3 are confluent at their 
bases, although a small vascular island, apparently belonging to 
the latter, appears detached. <A similar detached venous element 
lies between tributaries 1 and 2. These detached vascular areas 
are of common occurrence in later stages, as portions of the sec- 
ondary capillary plexus entering into the formation of the jugu- 
lar lymph sacs. Caudal to tributary 3 the presence of a small 
fenestra in the precardinal vein indicates the condensation of the 
perivenous plexus to form the beginning of a small secondary 
channel. Dorsal tributary 4, as in series 30 and 31 (figs. 22, 23 and 
24) is still the largest and most prominent of the series of pre- 
cardinal dorsal branches. As a whole it presents a condition pre- 
liminary to its differentiation into a medial dorsal somatic and a 
lateral veno-lymphatic component. In addition to its primary 
point of communication with the precardinal, at X in fig. 27, it 
now also opens caudad into the promontory. This condition, as 
