THE FIFTH AORTIC ARCH OF MAMMALIAN 

 EMBRYOS; THE NATURE OF THE LAST 

 PHARYNGEAL EVAGINATION 



FRANK REAGAN 



Contribution Number 121 from the Zoological Laboratory of Indiana University 



SIXTEEN FIGURES 



PART 1 



The question of the existence of six aortic arches in mammaUan 

 embryos has recently received much attention. In the minds 

 of some, perhaps, it is definitely and satisfactorily settled; yet 

 the known facts and the conclusions drawn from them are diverse 

 and conflicting. For this reason, anything further to be added 

 may be of interest. 



The significance of the question is well understood. A demon- 

 stration of six arches would serve to bring into line the branchial 

 circulation of mammals with that of the lower vertebrates, and 

 to strengthen the belief in the similarity of ontogenies in related 

 groups. By Lehmann and Locy the view has been advanced, 

 that, upon a demonstration of the same number of arches in all 

 air-breathing groups depends the comparability of the arches 

 giving rise to the pulmonary arteries. This view seems alto- 

 gether unnecessary since, if it be granted that mammals have 

 only five pairs of arches, all doubt of comparability should fall 

 on the more transitory and questionable vessels. To deny the 

 similarity of the pulmonic arches because of a variable number of 

 arches anterior to them, is as illogical as to doubt the identity 

 of the systemic arches because of the fact that different num- 

 bers of arches may exist posterior to them. Because of the con- 

 stancy of their characteristics, the identity of the pulmonic 

 arches of all air-breathing vertebrates is well established. 



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