504 FRANK REAGAN 



The dorsal portion of the V-shaped evagiiiation would appear, 

 then, to correspond to the transient fifth vessel, and to have lost 

 its double nature proportionately to the degeneration of the fifth 

 arch. The tendency towards a distinctness of division of the 

 'glandule thyroidienne' from the pre-pulmonic caecum closely 

 parallels the tendency towards the development of a perfect 

 'fifth' arch. 



The pre-pulmonic caecum, therefore, is branchial in nature to 

 the extent to which the pulmonic vessel is a true aortic arch; 

 what this extent is, must be determined by a more thorough 

 study than has yet been given to the question. Both seem to 

 have been greatly modified, if they have ever resembled closely 

 the parts anterior which have generally been considered their 

 homologs. 



From the foregoing considerations two views are possible: 

 (1) that the new arches so far exploited are merely irregularities 

 which happened to be suggestive of a fifth arch but have no sig- 

 nificance; and that the Y-shaped evagination is simply a fourth 

 pouch; (2) that the fifth arch occasionally exists and the intim- 

 ately related pharyngeal evagination represents more than a 

 fourth pouch. There is much to favor the latter alternative, 



CONCLUSIONS 



1 . The vascular irregularities are more or less common through- 

 out the branchial circulation and may in any part show a slight 

 tendency towards bi-lateral duplication. 



2. Even though this tendency is far more pronounced in the 

 region of the supposed fifth arch, the irregularities here should be 

 considered very reservedly, since in most instances they are 

 merely an expression of the tendency towards anastomosis, com- 

 mon to vessels in close juxtaposition. 



3. A fifth vessel, very closely approximating a theoretically 

 perfect aortic arch can be demonstrated for the pig; it may be 

 with or without connecting branches from either the systemic 

 or the pulmonic arch or from both. 



