Embryonic history of the aortic arches in mammals. 403 



the third aortic arches in the succeeding- stage. The walls of the 

 third arch appear reticulated, but there are no trabeculae extending- 

 across the lumen of the vessel. The small evagination from the 

 third arch, shown at x in Fig. 6, Plate 23, is present onlj^ on the left 

 side. The vessel running- forward from the dorsal end of the first 

 arch is flattened against the brain-wall and its breadth in recon- 

 struction gives an exaggerated idea its calibre. 



To sum up : degeneration in the first arch of this embryo takes 

 place after the manner described by Eathke for the second arch 

 in mammals. The vessel of the hyoid arch has probably reached 

 its maximum development in this stage, since in an embryo but 

 slightly older (not reconstructed) degenerative changes have begun 

 in this vessel. These changes are deserving of especial mention 

 since in the embryo next described a much later stage is shown. 

 The hyoid vessel in the unreconstructed embryo mentioned above as 

 degenerating, is still complete, but its lumen is comparatively small, 

 and in the mesoderm surrounding it, along almost its entire length, 

 small sinuses are present a few of which are connected with the 

 vessel itself. This condition indicates that in the degeneration of 

 this arch, a general decrease in the size of the lumen may take 

 place before the continuity of the vessel is interrupted. 



A comparison of the third arch in Fig. 6 with that of the 

 succeeding stage. Fig. 8, shows that very likely it is just established, 

 and has by no means reached its maximum development. The 

 evidence seems insufficient to call the evagination of the third arch, 

 shown in Fig. 6, a ventral bud of the fourth arch. 



Embryo of the eleventh day. The next stage to be 

 considered is represented in Fig. 7, Plate 22, by an embryo of eleveu 

 days development. The external view shows four well established 

 visceral arches. 



Fig. 8, Plate 23, shows a reconstruction of the aortic arches of 

 the left side of the same embryo. No trace remains of the first 

 arch except a slender vessel which passes forward into the mandibular 

 arch from the ventral end of the hyoid vessel. The two sides are 

 almost identical in this respect. The hyoid arch is incomplete, and 

 consists of a dorsal and ventral remnant. Of these, the dorsal 

 remnant is slightly longer than the ventral and the two are separated 

 from each other a little less than a third of the entire distance 

 between the dorsal aorta and truncus arteriosus. The walls of these 

 incomplete vessels are decidedly irregular. While the condition in 



Zool. Jahrb. XXII. Abt. f. Anat. 27 



