THE MAMMALIAN FIFTH ARCH 497 



Since the extreme irregularities back of the systemic arch render 

 the existence of a typical ''fifth" arch more doubtful, the follow- 

 ing question naturally suggests itself: do such irregularities occur 

 in connection with the other arches; if so to what extent; and is 

 a tendency towards bi-lateral duplication of irregularities more 

 strongly displayed in the region of the new vessel than elsewhere? 



In order to determine these points and to throw as much light 

 as possible on the probable nature of this fifth vessel and its asso- 

 ciated pharyngeal parts, a study was made of one hundred and fifty 

 p^ embryos,^ between stages of eighteen somites and 15 mm. 

 Most of the embryos studied were between and including the 

 stages of 8.5 and 9.5 mm. 



In embryos smaller than 6 mm., marked irregularities were 

 rare. The first, second and third arches often showed blunt 

 protuberances and rough walls. These seemed more common 

 on each arch at the height of its development. 



In an embryo of 6.5 mm., a vessel of about one third the calibre 

 of the fourth arch (fig. 15) was found springing from the middle 

 of the anterior side of that arch, and returning to it near its dorsal 

 lumen. This smaller vessel is a trifle similar to that of the human 

 embryo of Zimmermann, except that it is found on the anterior 

 face of the fourth arch. The conditions in fig. 15 would indicate 

 that the vessel described by Zimmermann was merely a division 

 of the fourth arch. 



At the stage of 7 mm., short spurs were occasionally present 

 near the ventral end of the fourth arch, projecting caudally. 

 Short projections were common on all portions of the posterior 

 face of the fourth arch. Stages of 8 mm. showed shghtly more 

 irregularities in the region of the fifth vessel than in the preceding 

 stage. In a few 8.5 mm. embryos, slender dorsal connections 



' I am much indebted to Professors C. H. Eigenmann and F. Payne for direction 

 and helpful criticism of the work undertaken. Acknowledgment of indebtedness 

 is also due Professor Frank R. Lillie for permission to consult the embryological 

 collection of the University of Chicago, and Professor C. H. Spurgeon of Drury 

 College, for the use of his excellent private collection. Besides the embryos of 

 these collections, those of the embryological cabinet of Indiana University, a 

 number of series of my own preparation, many student preparations were also 

 examined. 



