Embryonic history of the aortic arches in mammals. 401 



Embryo of the ninth day (second half). No record of 

 the age was received with this embryo. It is estimated to be still 

 in the ninth day, though somewhat older than that of the preceding 

 figure. 



In external view (Fig. 3, Plate 22) three gill-arches are distinctly 

 outlined. 



Fig. 4, Plate 23, shows a reconstruction of the aortic arches of 

 the left side for this embryo. The arches present are a first pair, 

 an incomplete second, and a third pair, complete upon the left side, 

 but incomplete upon the right. The dorsal aortae are distinctly 

 separate throughout their course. It was impossible to represent 

 the truncus arteriosus clearly in the reconstruction, on account of 

 the sharp curve which it makes near its union with the roots of 

 the aortic arches. The first aortic arch is of more than twice the 

 diameter of the second and third. Its walls are smooth, and no 

 trabeculae are present in its lumen. A small branch passes forward 

 to the brain from the anterior side of the first arch, a short distance 

 below its union with the dorsal aorta. In the second pair of aortic 

 arches the condition is as follows: — on the left side there is 

 developed from the dorsal aorta a very short but comparatively 

 thick lateral bud. In the two sections below this bud no trace of 

 a blood vessel can be found in the hyoid arch; but in the third 

 section a small vessel appears, which, though irregular in its course 

 and outline, can be traced to the truncus arteriosus. The second 

 arch increases slightly in size toward its ventral end. The condition 

 of the arch upon the right side is somewhat diiferent : An extremely 

 small spur extends from the dorsal aorta, but fails to unite with 

 the slender vessel from the region of truncus arteriosus which rises 

 to the same level as the dorsal bud, but lies slightly nearer the 

 surface of the hyoid arch. The ventral portion of the blood vessel 

 is irregular, and a short distance above the truncus arteriosus, breaks 

 up into lacunae. These, however, do not extend completely to the 

 truncus and no ventral spur of the right second arch could be found. 

 The right and left third arches are also asymmetrical in their deve- 

 lopment in this embryo. The dorsal and ventral portions of the left 

 arch are distinct in outline, and relatively large, but in its middle 

 region the vessel becomes very slender, and toward the larger dorsal 

 segment, is reticulated in appearance. The vessel may, however, be 

 called complete. On the right side a dorsal and ventral bud are 

 present, and a slender vertical sinus intervenes between them, but 



