THE MAMMALIAN FIFTH ARCH 501 



like projection extended down behind it from the dorsal aorta. 

 Blunt protuberances were found similarly located in three 

 instances. 



Figs. 11, 12 and 13 show irregularities slightly suggestive of a 

 fifth arch, but they are situated so far dorsally and laterally that 

 they need not be considered seriously. Also, they form such 

 straight and direct communications, lacking the curvature 

 found in the typical arches, that I consider it mere conjecture to 

 suggest their having significance. Granting with Locy that 

 irregularities such as these serve to demonstrate the extreme 

 variability of the supposed vessel, it must be admitted that these, 

 and those to which he had reference cannot at the same time prove 

 a typical existence for the new arch. 



On the grounds of majority it has been held that a perfect 

 fifth arch has, for its distal termination, the dorsal extremity of 

 the pulmonic arch. It was this consideration that led Locy to 

 oppose the view of Lehmann (above stated and confirmed). 

 But it is evident that, in the pig at least, the fifth arch connects 

 the two aortae and lies between successive pharyngeal evagina- 

 tions; hence it conforms to the requisites of a typical arch. It 

 may be that conditions other than these represent the highest 

 degree of development attained to by certain forms, but such 

 forms can not be said to exhibit it in a theoretically typical man- 

 ner. On the other hand, it is only to be expected that the vessel 

 in question generally be found in an a-typical condition and very 

 exceptionally perfect. Therefore an occasional constancy in 

 atypical conditions should not be allowed to dominate our con- 

 ception of a theoretically perfect arch. 



The relatively late appearance of the supposed fifth arch is 

 another point which believers in that vessel have seen fit to neg- 

 lect. This point receives consideration in Part 2. 



