64 H. D. SENIOR 
The abdominal plexus has no connection whatever with the 
segmental arterial series. It is drained partly by postcardinal 
plexus, but mainly by the umbilical vein. The pelvic plexus arises 
by two roots from the convexity of the umbilical artery opposite 
the roots of the abdominal plexus. It covers the cloaca and 
receives the pudendal artery and a large branch from the second 
sacral segmental. It is drained by the postcardinal plexus. 
The abdominal and pelvic plexuses appear to be connected by 
a few vessels passing lateral to the umbilical artery: It is 
probable that all the visceral branches of the adult a. hypo- 
gastrica (excepting the superior vesical) arise out of the primitive 
pelvic plexus of the original umbilical artery. The parietal 
branches (as far as they are present before the stage of 22 mm.) 
arise from, or very near to, the axial artery. They are therefore 
connected with the dorsal root.‘ 
This stage agrees in most essentials with the 5- -mm. embryo 
(II) described by Tandler (’03). The inferior mesenteric artery 
arises opposite the first lumbar segmental. 
b. Stage of 8.5 mm., C. U. C., no. 9, figs. 2 and 9 B 
The femoral, obturator, genitofemoral, and peroneal nerves 
are readily recognized as short relatively unbranched trunks. 
The extremity of the tibial is divided into what appear to be 
the medial and lateral plantar nerves. 
The aa. umbilicales have lost their original (ventral) roots 
and now spring from the aorta in the intervals between the 
fourth and fifth dorsal pairs of lumbar segmental arteries.» The 
4 The distinction between the respective origins of the visceral and parietal 
branches of the a. hypogastrica has already been pointed out by Wes Mle 
(704). 
5 It is questionable whether the eommon iliac artery should be regarded as 
belonging to the fifth lumbar dorsal segmental or not. _ At the stage of 8.5 mm. 
of this series the fifth lumbar segmental has regained its independence, and 
springs from the aorta (on both sides) between the fourth and fifth. The work 
of Levy (’02) shows, however, that absence of the a. lumbalis ima is the rule 
rather than the exception. Whether this vessel is frequently retained by the 
root of the a. iliaca communis or merely overshadowed (as it were) by it is 
doubtful. 
