AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 61 



clearly not to be regarded as late stages of the arches. They have not the size or 

 form of the arches. Functionally, also, they differ. Since they are interrupted 

 below, evidently their current is usually downward from the dorsal aortae. They 

 serve to supply the substance of the visceral arches and not to convey the blood- 

 stream from heart to aorta. 



The vascular successors of the arches remain but a short time and are in turn 

 replaced by slender vessels, which, except near their origin from the dorsal aortse, 

 are scarcely more than capillaries. These run close to the caudal confines of the 

 first and second visceral arches. These two pairs of successive vessels may be 

 termed, respectively, the earlier and later hyoid and earlier and later mandibular 

 arteries from the visceral arches which they supply. The later hyoid and mandib- 

 ular arteries are both present in the period between the establishment of the fourth 

 and sixth arches. In the branchial period, after the completion of the pulmonary 

 arch, the upper end of the later hyoid vessel seems always to be present. It is still 

 clearly distinguishable in the post-branchial period (plate 3, figs. 37 to 39). It is 

 the equivalent of the stem of Broman's (1898) hyostapedial artery in man. Tandler 

 has described in detail the development of the stapedial artery in the rat by the 

 capturing of branches from the upper end of the first arch by the upper end of the 

 second arch. There can be little doubt that the "arches" he refers to are the 

 earlier or later mandibular and hyoid arteries of the foregoing account. He finds 

 that the upper end of the "second arch" moves caudally a short distance along 

 the dorsal aorta. This we recognize as the later hyoid artery, which we know 

 has a slightly caudal position as it comes off from the dorsal aorta, due to its 

 passing down the caudal side of the visceral arch. In 13 and 14 mm. human 

 embryos this vessel has increased in caliber, keeping pace with the expansion of 

 this region in connection with the development of the ear. Tandler also finds it in 

 the human and identifies it as the stapedial artery. The development of its 

 branches and its later history were not followed in the present study. 



At the time the stapedial artery is developing in the hyoid arch, the precursor 

 of the external carotid is taking form on the ventral side of the propharynx. While 

 the second arch is disappearing, a pair of symmetrical arterial sprouts is usually 

 distinguishable, extending forward from the aortic sac in the region earlier occupied 

 by the ventral segments of the first two arches. In the two specimens showing this 

 stage these sprouts he ventral to the thyroid gland, and in one of these the distal 

 branches of the right sprout have been captured by the opposite vessel. Later (fig. 3) , 

 after the second arch has gone, these ventral primitive arteries are found to be on 

 either side of the thyroid gland. Each sends out a ventral branch to the plexus 

 of the pericardium and integument, and also a dorsal branch, which either breaks 

 up in the rich plexus of the thyroid gland or extends for a variable distance 

 through the subpharyngeal plexus toward or into the base of the mandibular or 

 hyoid visceral arches (fig. 4). 



An interesting feature of the adjustment of the ventral pharyngeal vascular 

 channels is the occurrence of small vascular enlargements in the subpharyngeal 

 plexus or at times in the ventral primitive arteries. These are termed lacunae 



