Development of the Jugular Lymph Sacs. pag 
spindle JV, and by a detached vascular island not distinctly 
joined to the main vein, or to the tributary complex to which it 
belongs. 
Dorsal tributary 4 (4, fig. 22) is large and typically developed. 
It is formed by the confluence of two extended paraneural channels 
(a and b) converging from the cephalic and caudal directions. 
Their confluence already exhibits the irregular dilatations charac- 
teristic of veno-lymphatic formation. The main right-angled 
trunk receives in addition a cephalic branch (4S), which represents 
the element of precardinal tributary 4 so often found in subse- 
quent stages entering the promontorio-precardinal junction on 
its medial aspect as the first of the series of promontorial dorsal 
somatic tributaries (4S, figs. 11 and 12). 
The precardinal spindle V is only moderately indicated. On the 
other hand, the distal enlargement VI is well marked, representing 
the anlage of the future jugular promontory. It marks the con- 
fluence of the pre- and posteardinal veins, which are still in a 
direct line, and gives origin ventrally to the duct of Cuvier, receiv- 
ing just cephalad of this point a short branch of considerable 
size which probably represents the rudiment of the definite 
external jugular vein. 
The dorsal region of this area is most interesting and suggestive 
of the later developmental stages. By the formation of a large 
fenestral space in the dorsal portion of the postcardinal, opposite 
to the Cuvierian confluence, an irregular dilated venous plexus 
_(H) has been produced which projects from the vein in the area of 
the future promontory. This plexus represents the anlage of 
the dorsal veno-lvmphatic plexus. 
Later, with the further development of the promontory and the 
resulting dorso-ventral arching of the postcardinal, the dorsal 
veno-lymphatic fenestrated area gains the level of the promontory 
and rides on its cephalo-dorsal aspect. 
Cephalad of this main fenestral arch H a second smaller arch 
(K) is developed in a similar manner from the dorsal circumference 
of the precardinal. This arch (K) probably represents, in part, 
the capillary plexus involved in the formation of the para-pre- 
cardinal channel of later stages (fig. 9). 
