Development of the Jugular Lymph Sacs. 231 
series 30 and 31 have become established. They are apparently 
derived from the antecedent type shown in series 134 and 47. 
These are the earliest stages which we have completely recon- 
structed, and they show the building up of the primitive main 
systematic venous system by the confluence of at first three and 
then of four principal channels which unite to form the duct of 
Cuvier and open through the same into the sinus venosus. 
Series 134, 5°” Embryo 
Reconstruction of left side, 
Lateral aspect, fig. 25 
In series 134 the duct of Cuvier is formed through the con- 
fluence of the precardinal, the posteardinal, the omphalo-mesen- 
teric and the umbilical veins. 
The omphalo-mesenteric vein, receiving a number of small trib- 
utaries from the pre-aortic region, possibly subcardinal radicles, 
arches dorso-ventrad and joins the precardinal to form the duct of 
Cuvier. The proximal dilated end of the omphalo-mesenteric 
receives on its lateral aspect the terminal of the umbilical vein. 
The latter is a vessel of large size, receiving by numerous tribu- 
taries the drainage from the body walls and the anterior limb 
bud. Along its dorsal circumference it is irregularly dilated and 
fenestrated. ; 
The precardinal forms a short, strongly curved arch, receiving 
anteriorly several short tributaries. Near its posterior extremity 
it receives dorsally two very large irregularly dilated branches 
(4 in fig. 25). Comparison with the succeeding stages (series 30 
and 31, figs. 22 to 24) suggests that they represent the elements 
of precardinal tributary 4. 
The postcardinal vein begins anteriorly in a dilated and fenest- 
rated extremity (H) which closely approaches the large dorsal 
tributaries of the precardinal but does not communicate with them. 
Near its anterior extremity the posteardinal, by a ventrally 
directed branch (LZ), establishes its connection at the cardinal- 
Cuvierian junction. Along the rest of its course it follows closely 
