The Pharyngeal Pouches in the IMammalia 241 



presence in certain others, it seems to be that this thymus IV cannot 

 in the mammals have the significance Verdun attributes to it. At least, 

 to my mind, the actual facts do not bear it out, whatever its theoretical 

 support may be. 



The lateral thyroid Verdun regards as an autonomous structure, rep- 

 resenting the post-branchial bodies of lower vertebrates. To me it 

 appears that this is done for the most part on purely theoretical 

 grounds. So far as actual facts are concerned, the lateral thyroid in 

 the mammals closely follows the thymus in its behavior, and I can see 

 no clear and convincing reason for regarding it as other than the ventral 

 diverticulum of the fourth pouch and therefore homodynamous with 

 the thymus of the preceding pouch. I Ivnow of no decisive evidence 

 against the view advocated by Verdun, but the facts adduced by him 

 are not sufficient to establish his point. I therefore follow the usage of 

 most writers in regarding the lateral thyroid as a part of the fourth 

 pouch. 



REFERENCES. 



1. Bell, E. T. The Development of the Thymus. Amer. Jour of Anat., 



Vol. V, 1905. 



2. BoEN. Ueber die Derivate der embryonalen Schlundbogen und Schliind- 



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3. FiscHELis, P. Beitrage zur Kenntniss der Entwicklungsgeschichte der 



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