AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 75 



conclusion. A progress of fusion from an intermediate point forward and backward 

 is in fact to be anticipated from the relation of the primitive paired aortse just 

 previous to the beginning of the process, as they show a region of closer approxima- 

 tion from which they diverge both cranially and caudally. 



A period of fixity in the position of the cranial point of fusion of the aortse 

 relative to the pharynx begins with embryo No. 810 and indicates that fusion in a 

 cranial direction has been completed. The bifurcation remains stationary until 

 the aorta begins to shift caudally relative to the digestive tube and respiratory 

 tract. The cause of the arrest of fusion here is to be found in the active separation 

 of the vessels due to the pressure exerted by the expanding rudiments of the ver- 

 tebrae and esophagus, between which they he. As will be seen, a separation of 

 this nature is not unique for this region, but is much better marked in a more 

 cranial part of these vessels. 



When fusion ceases, the bifurcation of the aorta is approximately opposite the 

 seventh body segment. Relative to the nerve-tube, this point now lies more cau- 

 dal than it did at an earlier period, due to the fact that a forward shifting of the 

 cranial end of the nervous system relative to pharynx and aorta has been taking 

 place more rapidly than the cranial progress of the point of fusion. In this way it 

 comes about that in embryo No. 1075, for example, fusion, though still progressing, 

 is opposite the second cervical ganglion, while in No. 810, in which the unpaired 

 aorta is complete, it is opposite the seventh. In embryo Strahl 10, of the Keibel 

 and Elze (1908) table, the aortic bifurcation is also given as opposite the second 

 cervical ganglion. 



The approximation of the walls of the primitive aorta; in an intermediate 

 region results in the existence of caudal paired aortas for a time after an unpaired 

 aorta has become established. They are never long vessels, because, while they are 

 extending caudally by their differentiation from a plexus, they are shortening at 

 their cranial end by fusion. The paired condition, except possibly in the form of 

 slender terminals, does not remain in this region, as at the cranial end of the embryo. 

 In 4 and 6 mm. embryos only very short double vessels are present. In other em- 

 bryos, ranging from 5 to 18 mm., the vessel is seen in section to be single, at least 

 until it has shrunken to a very small caliber. 



At the time the primitive aortae are fusing they are continuous with paired 

 longitudinal neural arteries which pass backward under the brain. In the formation 

 of the basilar portion of these, which is terminated cranially by the region of the 

 hypophysis, there is a fusion much like that of the aorta. There is left a segment in 

 the forebrain region which, like the cranial end of the paired aortae with which they 

 are continuous, does not fuse. On the contrary, the two originally parallel vessels, 

 each with its longitudinal neural and carotid parts, are carried away from each 

 other to a greater or less degree in various regions, depending upon the activity 

 of the lateral growth of the surrounding structures. In the late branchial period 

 they have three well-marked regions of divergence. These reach their maximum 

 opposite the middle cervical, the anterior pharyngeal, and the diencephalic regions, 

 respectively. 







