LEPIDOSIllEN PARADOXA. 
Ill 
the pi-onepliric tubules, where it is impossible to trace them 
among the sinuses of those structures. It is possible that 
the glomerular arteries may establish connections with the 
vitelline meshwork for a time, through the pronephric sinuses. 
At Stages 31-32 the developing liver comes in contact with 
the ventral surface of the right glomerulus and pronephric 
tubules and a small artery passes from the tip of the glomerulus 
along the right outer angle of the liver, and, crossing ventral 
to that organ, reaches the right wall of the developing gut. 
At Stage 36 with the atrophy of the glomeruli and pronephric 
tubules, the left glomerular artery disappears, but the right 
artery persists as the coeliac artery. In Lepidosiren 
apparently there is only one vessel to each pronephric 
glomerulus, and the cceliac artery receives no branches either 
directly or indirectly from the aorta, and only anastomoses 
with the anterior mesenteric artery in the walls of the intestine. 
There is no evidence of any shifting of the root of this vessel 
during development. 
Pulmonary Arteries. — The pulmonary arteries appear 
about Stage 31 +, arising from the sixth aortic arch on either 
side. When the sixth aortic arch dwindles in size, as already 
described, the pulmonary artery comes to open from the 
common stem of the fifth and sixth aortic arches, receiving 
its main blood-supply from the fifth. With the rotation of 
the lungs, the right pulmonary artery comes to be distributed 
to the dorsal and the left to the ventral surfaces of the lungs. 
The segmental aortic vessels from which the mesonephros 
receives its arterial blood-supply appear about Stages 31, 32, 
as do also tlie subclavian, pelvic and mesenteric arteries. 
V. Development of the Veins. 
Ductus Cuvieri. — At Stage 24 -f (twenty-four segments) 
the endothelial rudiments of the vitelline veins extend, pos- 
terior to the pericardium, one on either side, across the 
anterior surface of the yolk, outwards and somewhat pos- 
teriorly (Text-fig. 24 A, F. F.). 
