A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 157 



Over the yolk-stalk the tributaries of the vitclline veins form a network of 

 small vessels. These reach dorsally as far as the middle of what might be .outlined 

 as the gut. Extending along the gut-wall, therefore, are a number of small 

 longitudinal coursing veins (plate 5, fig. 1). They can be traced caudally along 

 the hind-gut for some little distance below the hind-gut portal. The significance 

 of this plexus I have not been able to determine definitely, but I judge that it will 

 ultimately give rise to the inferior mesenteric vein. 



SINUS VENOSUS. 



The sinus venosus is situated within the substance of the septum transversum, 

 ventral to the gastric region of the fore-gut and cephalad to the tip of the hepatic 

 diverticulum. It is a broad, irregular vessel (text-fig. 4 and plate 5, fig. 1), much 

 flattened dorso-ventrally. On its left border it receives three vessels which repre- 

 sent the left vitello-umbilical trunk. On its right it receives three small branches 

 from the right vitello-umbilical trunk and as many more directly from the right 

 vitelline vein. The sinus venosus curves ventrally and to the left and, becoming 

 a more rounded vessel, passes out of the transverse septum into the atrium of 



the heart. 



ARTERIES. 



AORTA VENTRALIS. 



The ventral aorta (plate 2, fig. 3, and plate 3, figs. 3 and 4), the direct con- 

 tinuation of the truncus arteriosus, is an unpaired median vessel. It is situated 

 just ventral to the thyroid diverticulum, with which it lies in contact. It at once 

 breaks up into the aortic arches. 



AORTIC ARCHES. 



Three pairs of aortic arches are present (plate 5, fig. 2). Of these the first is 

 by far the largest. Each vessel begins at the anterior extremity of the ventral 

 aorta and extends anteriorly and dorsalward in the first visceral arch, just cephalad 

 to the first pharyngeal pouch. Reaching the upper extremity of the arch, it 

 joins with the dorsal aorta. Both first arches are distinctly patent throughout. 



The second and third aortic arches are smaller and less distinct vessels, lying 

 within the second and third arches respectively. The vessels of these arches vary 

 in size and distinctness in different regions. In certain places they become so 

 small that a lumen is no longer discernible, and it becomes impossible to determine 

 whether the vessels are continuous or not. It is very probable, however, that 

 connections do exist at these places, but, owing to the great similarity between the 

 endothelial cells and those of the surrounding mesenchyma, the former can not be 

 traced through with any degree of certainty. On the left side the second arch 

 shows two such doubtful interruptions, one at the point where it leaves the ventral 

 aorta, the other where it joins the dorsal aorta. Between the two breaks a distinct 

 vessel is present. The third arch on the left side arises from the dorsal aorta as 

 a small plexus and extends as a distinct vessel halfway down to the ventral aorta. 

 Here it disappears for several sections, but soon reappears as an apparently solid 

 string of endothelial cells. This cord when traced downward is found to connect 

 with the ventral aorta. On the right side the second and third aortic arches like- 



