457 
Table III. 
Vertebral Levels of Organs in Embryo H 60 (11 mm). 
Organs POT: “Vertebral Level 
Heart 6th cervical to 4th thoracic 
Formation of dorsal aorta | between 7th cervical and lst thoracic 
Origin of subclavian art. | 7th cervical 
Ducts of CUVIER 7th cervical 
Origin of vitelline art. 8th thoracic 
Oesophagus 5th cervical to 3rd thoracic 
Stomach 3rd thoracic to 7th thoracic 
Liver 3rd thoracic to 7th thoracic 
Trachea 5th cervical to Ist thoracic 
Lungs 1st thoracic to 3rd thoracic 
Thyroid gland 5th cervical 
Suprarenals 3rd thoracic to 6th thoracic 
Wolffian bodies 2nd thoracic to lst lumbar. 
Certain features concerning the blood vessels in this embryo de- 
serve special mention. The dorsal aorta is formed by the union of 
the two lateral aortic arches, just below the origin of the subclavian 
(vertebral) arteries. At its origin, the aorta is comparatively narrow 
in caliber (Figs. 2 and 3), but gradually enlarges as it passes down- 
ward, until its diameter becomes at least twice as great towards its 
lower end (Fig. 3). The hypogastric and caudal arteries are seen to 
be wide at their origin, but soon diminish rapidly in caliber (Fig. 3). 
The vitelline artery in this embryo arises from the aorta opposite 
the 8th thoracic vertebra, and runs downward in the mesentery of 
the U-shaped loop of the intestine. On reaching the extremity of the 
loop, the artery divides into two branches which encircle the intestine, 
uniting again into a single trunk at the attachment of the yolk stalk 
(Figs. 2 and 3 ys). HOocHSTETTER (7) has described a similar ring 
formed by the vitelline artery in mammals (cat). 
In earlier embryonic stages, as is well known, there exist two 
distinct vitelline arteries passing out, one on each side of the intestine, 
to reach the yolk sac. One of these (usually the left) is said to 
atrophy, leaving the other as a single vessel. The arterial ring existing 
in this case does not agree with this theory, however. Possibly the 
single trunk is formed by a fusion of the two primitive vitelline arteries 
except where they persist to form this intestinal ring. Later, one side 
of the ring evidently atrophies, leaving a single vitelline artery. Still 
later, all of the vitelline artery distal to the intestine atrophies with 
the yolk sac, the proximal portion persisting as the superior mesenteric 
artery of the adult. 
The vitelline (omphalomesenteric) vein crosses above the intestine 
at the attachment of the yolk stalk, and enters the mesentery, form- 
