404 Harriet Lehmann, 



these incomplete arches is very similar on both sides, there are 

 present, only upon the left side, numerous sinuses not connected 

 with the vessel, but lying in the surrounding mesoderm near the 

 outer surface. These sinuses extend from the dorsal aorta to the 

 truncus, but are less numerous in the region midway between those 

 vessels. 



The third arch in this embryo is greatly increased in size, and 

 is relatively larger than at any other stage observed. The fourth 

 is less than half the diameter of the third, and its walls are not 

 entirely uniform. From the ventral end of the fourth arch upon 

 both sides, a spur extends dorsally, and receives a very slender 

 vessel (x) which has taken its course along more than half the 

 length of the fourth arch, and a short distance exterior to it. 

 Opposite this ventral spur, the dorsal aorta shows a distinct 

 evagination from its ventral side. These structures may be compared 

 with those present between the fourth and sixth arches in the 

 succeeding stage (Fig. 10, Plate 23). A comparatively large ventral 

 bud of the sixth arch is present, behind the one just mentioned, but 

 no corresponding dorsal element could be found in this stage. 



The left dorsal aorta is sliglitly larger than the right, especially 

 below the region of the aortic arches. The subclavian arteries, not 

 included in the reconstruction, are present in this embryo, and arise 

 from the dorsal aorta in connection with segmental arteries, at a 

 considerable distance behind the union of the aortic roots. No 

 distinct subclavian artery was found in the earlier stages described. 



The ventral rudiment of the first arch which persists in this 

 stage, corresponds in position to the ventral remnant in the preceding 

 stage. The persistence of such a structure in embryos in which the 

 hyoid aortic arches are only partly degenerated agrees with Eathke's 

 statement for mammals. The manner of disappearance in the second 

 arch is also in accordance with Eathke's description, the middle 

 portion being first to degenerate. The sinuses mentioned as lying 

 in the surrounding mesoderm, are probably remnants of the earlier 

 large hyoid vessel. (I have considered the possibility of this being 

 a phylogenetic remnant of a capillary connection between the efferent 

 and afferent vessels, but is seems to me unlikely, since lung-breathing 

 vertebrates have no functional gill in the hyoid arch.) The third 

 arch has probably reached its maximum development in tins stage, 

 since a half day later it is relatively much smaller. No stages at 

 my command showed the fourth arch in the actual process of 



