590 CHARLES EUGENE JOHNSON 



DISCUSSION 



The evidence presented above shows that in birds a fifth vis- 

 ceral pouch is developed which is a different diverticulum from 

 that which gives rise to the ultmiobranchial body. This fifth 

 pouch and the ultimobranchial diverticulum are strictly com- 

 parable to and homologous with the fifth pouch and the ultimo- 

 branchial divert iculmn of reptiles. 



For the lizards, Peter ('00) and Saint-Remy et Prenant ('03- 

 '04) in more recent years have demonstrated the presence of five 

 visceral pouches, exclusive of the diverticulum giving rise to the 

 ultimobranchial body. In Anguis fragilis, Lacerta viridis, and 

 L. agilis, Saint-Remy et Prenant found that a relatively small 

 fifth pouch is developed, which, however, soon disappears without 

 leaving any trace. The left ultimobranchial body alone under- 

 goes progressive development while the right usually vanishes, 

 as in birds. 



In the snakes these same authors also found five visceral 

 pouches, but the fifth pouch here gives rise to a portion of the 

 thymus and to a parathyreoid body. The ultimobranchial body 

 persists on both sides. 



In the turtles, van Beimnelen ('93) found that in Chelonia 

 viridis (mydas) the fourth and fifth pouches are developed simul- 

 taneously with the ultmiobranchial diverticulum from a lateral 

 'blinddarmformigen Falte' at the posterior end of the pharynx. 

 These pouches and the ultmiobranchial anlage soon become 

 pinched off from the pharynx and form a complex of three united 

 vesicles. 



In embryos of Chelydra serpentina I find that the structures 

 in question arise in a smiilar way to those of Chelonia viridis. 

 In an embryo of 6 mm. the fourth and fifth pouches and the 

 ultimobranchial diverticulum can be recognized in the fonii of 

 three secondary outpouchings from a relatively large globular 

 primary evagination of the pharynx. By the 9-mm. stage a deep 

 constriction has appeared extending dorsoventrally in the trans- 

 verse plane and separating the fourth pouch on the one hand 

 from the fifth pouch and the ultimobranchial anlage on the other; 



