264 Proceedings of the Royal Physical Society. 
towards the head and gives off three aortic arches on each side (Fig. 2), 
These are the vessels supplying visceral arches II, III. and IV. There are 
traces of another vessel, that of the first arch in front of these. The vessels 
supplying the second visceral arch are slender, and are in process of disap- 
pearing. Those of the third visceral arch are larger. They, and those of 
the second arch, come out of the ventral aorta by a common stem, The 
right and left fourth aortic arches, the largest vessels, are united at their - 
exit from the ventral aorta and run together for some distance in line with 
the proximal portion of the ventral aorta. The left vessel is larger than the 
right, foreshadowing its development into the definitive systemic aorta. 
Vv. a. 
Fra, 2.—Aortic Arches: Diagrammatic drawing from reconstruction 
as seen from the right side. 
a.7., aortic roots ; v.a., ventral aorta ; ZZ, TJ, 1)’, aortic arches, 
The aortic roots unite about the level of the fore limbs and run back as 
the dorsal aorta, eventually dividing into two iliac arteries. Two umbilical 
arteries are present, one coming off the dorsal aorta on the left hand side, 
and the other off the right iliac. These run into the umbilical cord and 
unite in the placenta, eventually splitting up and joining a network of 
smaller vessels at the surface. 
I am glad to take this opportunity of thanking Professor Agar for 
placing the material at my disposal, and for his help in working it out. To 
Professor Walmsley I am indebted for the use of his laboratory in Glasgow. 
The work was carried out in the laboratory of Professor Graham Kerr, 
who very kindly assisted me and gave me every facility for work. 
(Issued separately, 20th June 1921.) 
