502 FRANK REAGAN 



PART 2 



The double pharyngeal out-pocketing found back of the sys- 

 temic arch is not well understood. At present there are three 

 views concerning its significance: (1) that the double outpocket- 

 ing represents the fourth pharyngeal pouch with its dorsal and 

 ventral diverticula (Fox, Tandler, Coulter); (2) that the dorsal 

 and ventral parts of the outpocketing represent the fourth and 

 fifth pouches respectively (Greil, Zimmermann) ; (3) that the ven- 

 tral portion represents the postbranchial body of lower verte- 

 brates (Mauer, Verdun). From these considerations the ventral 

 portion of the double outpocketing has been designated by the 

 following terms: (1) ventral diverticulum of the fourth evagina- 

 tion; (2) fifth branchial pouch or ultimo-branchial body; (3) 

 post-branchial body. Since no one of these terms is sufficiently 

 general to cover all the views presented, the term 'pre-pulmonic 

 caecum' might be suggested. This term is applicable to the 

 ventral division of the out-pocketing, whatever may be its signif- 

 icance. The further usefulness of the term is shown later. 



This much, however, is definitely settled: the dorsal portion 

 of the double pouch is the anlage of the 'glandule thyroidienne' 

 of Prenant, while the 'pre-pulmonic caecum' becomes the rudi- 

 mentary lateral thyroid. 



According to Verdun the development of the last pharyngeal 

 evagination (or evaginations) is as follows: the pre-pulmonic 

 caecum is first given off as a latero-ventral diverticulum, posterior 

 and mesial to the third pouch. Near the dorsal extremity of the 

 caecum, and slightly anterior to it the ''glandule thyroidienne" 

 is given off immediately from the pharynx as an independent 

 diverticulum. Both push out laterally, drawing with them the 

 wall of the pharynx lying between their bases. Thus they acquire 

 secondarily a common pharyngeal orifice. Their formation differs 

 in this last respect from that of the pouches anterior. 



Fox ('08) in describing the posterior evagination of a 6.5 mm. pig, 

 gave the location of the ' dorsal portion of the fourth pouch' as posterior 

 and dorsal; he gave the 'ventral portion' as anterior and ventral. This 

 may have been due to his considering directions with reference to the 

 general contour of the embryo, and not with reference to the direction 

 of the pharynx where the evagination joins it. 



