A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 159 



branch terminates near the mid-line of the embryo not far from its fellow of the 

 opposite side. In addition, the anterior branch gives off along its course two or 

 three small arterial twigs. These pass dorsally along the wall of the brain, and 

 probably represent the beginning of the cerebral arteries. 



Dorsal Segmental Arteries. The dorsal segmental arteries are represented by 

 24 paired vessels. Although for the most part they are similar in position and 

 distribution, they vary greatly in size and in the distinctness with which they can 

 be traced. The following description refers only to those of the left side. 



In his account of these vessels of the Robert Meyer embryo No. 300, Evans 

 states 8 : 



"At this stage the dorsal segmental vessels form in the tissue of the intersomitic 

 clefts large, well-marked vascular arches or loops, ono limb of which is against the neural 

 tube, while the other joins the cardinal vein." 



Evans 7 had previously shown similar loops in chick embryos. 



In my embryo I find that loops are present or indicated in case of all of the 

 upper 18 dorsal segmental arteries except the first. Two such typical vessels are 

 shown in plate 6, figure 2. Each dorsal segmental artery extends dorsally and 

 laterally from the aorta. When it reaches a point about halfway up the medullary 

 tube it branches, sending a short branch mediallly and a longer one laterally. The 

 lateral branch extends through the intersomitic cleft and reaches the posterior 

 cardinal vein as described by Evans. The medial one, I find, soon divides into two 

 distinct smaller branches, a dorsal and a ventral. These extend along the wall of 

 the medullary tube and tend, with their fellows of the opposite side, to encircle it. 



The first dorsal segmental artery has a relatively extensive origin from the 

 dorsal aorta. It extends dorsalward between the first and second somites, where 

 it breaks up into several branches. One of these extends for a short distance 

 anteriorly, while others extend caudally toward the second dorsal segmental. 

 A connection between the two, however, is apparently not yet formed. I am 

 also unable to trace a connection between this artery and the anterior cardinal 

 vein. In Ingalls's specimen of 4.9 mm. the first dorsal segmental artery (known 

 also as the hypoglossus artery) likewise sends branches both anteriorly and 

 posteriorly. The posterior branch, however, has joined the second dorsal seg- 

 mental artery, while the anterior is much longer and is easily recognized as the 

 arteria vertebralis. In my specimen, therefore, one sees the very beginning of the 

 formation of the vertebral artery. 



The second and third dorsal segmental arteries show only lateral and dorsal 

 branches, while for the fourth only pieces of a typical dorsal segmental artery 

 could be identified with certainty in the sections. It seems improbable that this 

 and other similar vessels are actually incomplete, more probable that they are 

 complete and that in places are so small and indistinctly differentiated from the 

 mesenchyma that the connecting portions have been overlooked. The lateral 

 branches of the second, third, and fourth dorsal segmental arteries unite with the 

 anterior cardinal vein; those of the remaining dorsal segmental arteries join the 



