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



This embryo is unique in two particulars. First, it is the youngest embryo recorded 

 in which there is any fusion of the dorsal aorta?. Those nearest to it and younger, such as 

 that described by Low, have still no fusion whatever, while in those just older, such as 

 described by Thompson and Van den Broek. the dorsal aortse are fused over the yolk sac. 

 In Thompson's case the somites at the level of fusion are given, and include only one more 

 than in Embryo VI, in his case being from the seventh to the thirteenth, in this case the 

 eighth to the thirteenth. The second unique feature is the second fusion of the aortse in 

 the caudal region, where even in older embryos paired aortse are found. It will be noticed 

 also that in the region between fusions the aortse lie very close together, only the narrow 

 notochord lying between them. The aortse show their largest caliber around the origin 

 of the umbilical arteries and just posterior to this. 



The branches of the aortse are numerous. There is no sign of forward extension of 

 any artery into the head as the internal carotid on the right side, but there is a short branch 

 on the left (plate 1; plate 3, fig. 1) continuing forward in the line of the dorsal aorta to a 

 point just in front of the hypophysis cerebri. There is a dorsal intersegmental artery 

 (plate 1) between every two somites back to the depth of the concave dorsal bend. Pos- 

 terior to this point the sections cut the body in a plane unfavorable for finding these arteries, 

 and it can not be stated whether they are present or not. There are many primitive 

 vitelline arteries (plate 1) given off as ventrolateral branches both of the paired and 

 unpaired aortse, and these ramify over the wall of the yolk sac. The roots of the umbilical 

 arteries are in series with these. Dorsolateral branches of the aorta run to the pronephros 

 and cranial half of the mesonephros. Arteries to the gut and urinogenital system were 

 not counted, as they were hard to detect and in some cases, where the shape of the aorta 

 indicated the origin of a branch, the branch could not be followed. Counting would thus 

 not give any accurate number and so was abandoned. 



The umbilical arteries run out into the belly stalk, one on each side of the allantois, 

 and unite dorsal to the allantois, as they all turn into the stalk. This common vessel is of 

 immense size and filled with blood, forming a blood reservoir (plate 1), as in the embryo 

 described by Dandy (1910). A short distance out in the belly stalk the artery divides, 

 and then immediately unites again, forming a loop through which the allantois passes as it 

 proceeds from a position ventral to the artery to one dorsal to it and ventral to the veins. 

 The artery is now single all the way out to the chorion, where it divides into two, each of 

 which ramifies over one-half of the chorion, supplying the villi. 



The arteries in Embryo V are somewhat smaller and are much harder to follow than 

 in Embryo VI, part of the difficulty being due to their being so filled with blood in many 

 cases as to be almost indistinguishable from the surrounding mesoderm. There are two 

 branchial arch vessels on each side, the second of which is only just complete. The dorsal 

 aortse are fused from the ninth to the thirteenth segments, one less than in Embryo VI. 

 The origin of the umbilical artery on each side is from the fourteenth to seventeenth seg- 

 ments and the caudal aortse are fused over the cloaca as in the other embryo. The aorta 

 terminates here shortly before the end of the gut, allowing the persistence of that part of 

 the neurenteric canal between the notochord and the gut, as already described. 



VEINS. 

 The only veins as yet developed are the primitive embryonic trunks. 

 The vena capitis medialis (plate 1) is present on each side, beginning in the region of 

 the optic vesicles. It can be traced caudad, without any interruption, to just in front of 



