498 FRANK REAGAN 



between the fourth and puhnonic arches were observed. Pro- 

 jecting into the dorsal aorta and sometimes piercing it, 'islands' of 

 mesoderm were often found. There was a sUght tendency 

 towards a duplication of these conditions on opposite sides. 



An 8.6 mm. pig (measured after having been killed in Zenker's 

 fluid and dehj^drated), number 78. S of the C. H. Spurgeon col- 

 lection, is of unusual interest because of the remarkably well 

 developed fifth arch. The vessel exists typically on the right 

 side without dorsal connection with either of the neighboring 

 arches. ^ 



Figs. 6 and 7 are from wax reconstructions of the branchial circulation 

 of the embryo. They were made by the method of Born. The cavities 

 of the arches are represented as solid. The entodern of the pharynx 

 and the ectodern of a portion of the body wall are represented as such. 

 In maintaining perspective, some parts of the model may appear to 

 have been drawn out of proportion, but apparent discrepancies will be 

 accounted for by a comparison of figures. In fig. 7, for instance, the 

 intimacy of the fifth vessel with the ventral aorta would seem exag- 

 gerated, since it is impossible to show the extent of the ventral aorta in 

 a lateral view. On the other hand, fig. 6 hardly does justice to this 

 intimacy since the ventral diverticulum of the third pharyngeal pouch 

 conceals the point of diversion of the third and fourth arches. The origin 

 of the fifth arch, however, is definitely located when the sections (figs. 

 1 to 5) are consulted. 



Fifteen microns lateral to the plane containing the point of 

 diversion of the third and fourth arches, the most median and 

 ventral indication of the fifth vessel is seen as a low ridge- (fig. 5) 

 on the ventro-caudal surface of the ventral aorta. ^ The ridge 

 increases in size and follows for a very short distance the course 

 of the ventral aorta laterally and dorsally. Continuous with 



^ After the completion of the reconstruction another small island of mesodern 

 was found in this ridge, but is not shown in Fig. 6. 



' The common trunk of the fourth and fifth arches lateral to the diversion of the 

 third and fourth arches may still be considered as ventral aorta for the following 

 reasons: (1) the distance is only 15 microns; (2) for the same reason that the com- 

 mon trunk of the third and fourth arches isalways regarded as ventral aorta; (3) 

 greater intimacy of the arches is allowable at their points of origin than at their 

 places of termination since they must all arise from a common centre, necessitating 

 the existence of common trunks. But in no instances do the more stable arches 

 normally enter the dorsal aorta by common trunks. 



