No.2.] VENTRAL ABDOMINAL WALLS IN MAN. 363 
by the development of the intercostal vessels. In pigs’ embryos 
the extensive membrane is also free from veins, with the excep- 
tion of the zone encircling the cord, which again has a venous 
plexus more marked, however, than in the human embryo. 
It appears, then, that during the third week of development, 
while the umbilical veins still empty into the sinus reuniens, an 
extensive plexus is formed throughout the greater extent of the 
membrana reuniens, which receives blood from the aorta on its 
dorsal side, and empties it into the umbilical vein on its ventral 
side. As the umbilical vein changes its position to enter the 
liver, this circulation through the membrana reuniens is broken 
up as a much earlier circulation though the umbilical vesicle 
was broken up.! 
The earliest collecting vein for the descending aorta is the 
omphalomesenteric vein; next it is the umbilical vein, and finally, 
when the abdominal walls are completed, it is the cardinal. 
This in turn is partly converted into the vena cava inferior. 
The arterial system is already well outlined in embryo II. 
The aortic arches and segmental arteries are sufficiently well 
marked to number them. The vertebral artery is in process 
of development; it being formed by a union of a number of 
segmental arteries, as shown by His,? by Froriep,? and by 
Hochstetter.4 The sixth cervical segmental artery gives rise 
to the subclavian artery. The lower cervical, all the dorsal and 
lumbar segmental arteries, are concerned in the development 
of the thoracic and abdominal walls. 
Fig. 2 shows the general extent of the dorsal myotomes, 
while Fig. 15 gives the extent of the arteries. It shows a simple 
arrangement from the vertebral to the hypogastric artery. The 
lower lumbar arteries are not shown. A section of this embryo 
is shown in Fig. 3. It shows the relation of the segmental 
arteries to the myotome. The segmental arteries supply 
primarily the spinal cord and ganglia in two groups of 
1In Journ. of Morph., vol. xii, Fig. 16, it will be seen that the aorta sends a 
number of segmental arteries to the umbilical vesicle. In a short time these all 
disappear, only to repeat themselves over the body of the embryo. In turn this 
second set disappears as soon as the umbilical vein enters the liver. 
2 His, Anat. Mensch. Embryone, vol. iii, 1885. 
3 Froriep, His’s Archiv, 1886, p. 69. * Hochstetter, Morph. Jahrb., Bd. xvi. 
