YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 27 



The ventricle resembles a letter U laid on its side, so as to have dorsal and ventral limbs 

 and a flexure looking cephalad, and with the atrium lying over the open end. 



THE ARTERIES. 



From the unpaired ventral aorta, where it bulges up under the floor of the pharynx, 

 arise the right and left ventral aorta? (plate 4, fig. 1), just back of the second branchial 

 pouch. They diverge like the limbs of a letter V, and as they run forward form a promi- 

 nent ridge in the floor of each side of the pharynx. There is no definite end to them, 

 since each passes imperceptibly into the first branchial arch vessel, which is (plate 1 and 

 plate 3, fig. 2) almost as large as the ventral aorta from which it springs, and runs cephalad 

 and dorsally in front of the first gill pouch, and then arching caudally over the pouch turns 

 into the dorsal aorta. From the outer side of each ventral aorta arises the second branchial 

 arch vessel (plate 1 and plate 3, fig. 2), which is only half the caliber of the first, but is 

 complete, possesses a lumen, and communicates with the dorsal aorta by running dorsally 

 between the first and second gill pouches, in an arch convex laterally, while the first arch 

 was convex forward. There is a slight bud posterior to this, which is the first indication 

 of the third branchial artery. This embryo fits in perfectly in the series of early embryos, 

 in development of the aortic arches. Dandy's embryo (1910) of 7 somites possesses one 

 completely formed, the first arch, with small buds of others posterior to this. Low (1908) 

 in the embryo of 14 somites finds the first arch complete, the second being formed. Then 

 comes the embryo of 18-19 somites here described with two arches, the second just com- 

 plete, and immediately following is the embryo of 23 somites described by Thompson 

 (1907), which shows two complete arches. 



The two dorsal aorta? begin anteriorly, directly continuous with the first branchial 

 arch vessel, and are situated far out at the sides of the pharynx. As they run caudally 

 they very gradually approach each other, and finally, about half way back over the yolk 

 sac (plate 1) they fuse beneath the notochord, presenting an hour-glass shape as seen in 

 the cross-sections, showing that fusion has only just occurred. This area of fusion extends 

 from the eighth to the thirteenth somite, inclusive, and then the two aortse again separate, 

 but lie in close contact with the gut and notochord. At the level of the succeeding 4 

 somites are the roots of origin of the right and left umbilical arteries (plate 1) from the 

 aorta?. These arise from a series of large ventral branches of the aorta?, which lie alongside 

 of the gut, from the end of the yolk stalk to the beginning of the allantois. These all 

 anastomose and give rise to the umbilical artery. This origin is in accord with the latest 

 investigations of Felix (1912) and others. It will be seen that at this stage the umbilical 

 artery arises opposite the third to sixth thoracic segments. In two embryos studied by 

 Felix, one younger, one older, the origin of the umbilical artery is respectively cranial and 

 caudal to this. In the younger embryo, Pfannenstiel III, of 14 somites, the origin of the 

 umbilical artery is opposite somites 12, 13, and 14, then in this embryo of 18-19 somites it 

 is opposite the fourteenth to the seventeenth, and in the Meyer embryo of 23 somites is 

 opposite the unsegmented mesoderm posterior to the somites. At the commencement of 

 the unsegmented mesoderm in Embryo VI, over the beginning of the cloaca, the two dorsal 

 aorta? fuse a second tune, and form a vessel in the shape of an inverted letter U from here 

 to their termination, which is coincident with the end of the gut. This description applies 

 to the muscular tubes of the arteries. The endothelial tubes are very small, often indis- 

 tinct, and as far as can be ascertained do not fuse similarly to the muscular ones, so that 

 the endothelial aorta? are separate throughout their entire extent. 



