THE DEVELOPMENT OF THE THYMUS IN THE PIG 



I. MOEPHOGENESIS 



J. A. BADERTSCHER 



From the Department of Histology and Embryology, Cornell University, 



Ithaca, N. Y. 



TWELVE FIGURES — TWO PLATES 



HISTORICAL 



It has been definitely established by many investigators that 

 the thymus of all mammals is of epithelial origin. More recent 

 investigations have shown, however, that the epithelial anlage 

 of the thymus is not derived from the same germ layer in all 

 mammals. Investigators are agreed on the point that the thy- 

 mus of mammals, when it is of a purely entodermal origin, is 

 a derivative of the ventrally directed epithelial diverticulum of 

 the third pharyngeal pouch. It has also been quite definitely 

 settled that in some mammals (mouse, Roud '00) the thymus 

 is entirely of ectodermal origin. The mixed (ectodermal-ento- 

 dermal) origin of the thymus in some mammals has not yet been 

 generally accepted. In pig embryos it is the close topographi- 

 cal relation that exists between the cervical vesicle and the third 

 pharyngeal pouch that makes a mixed origin of the thymus 

 possible. 



Among some of the workers on the early development of the 

 thymus of the pig may be mentioned Fischelis, Kastschenko, 

 Zotterman, Born, Bell, and Fox, the first thi'ee of whom attribute 

 to the thymus an ectodermal-entodermal origin. Fischelis ('85) 

 derived the thymus from the third pharyngeal pouch and the 

 third branchial groove. According to this investigation these 

 two fuse, and from then- point of fusion each contributes about 

 one-half to a ventrally directed downgrowth, the anlage of the 

 thymus. This conclusion is erroneous, for that portion of the 



317 



THE AMERICAN JOURNAL OP ANATOMY, VOl.. 17, NO. 3 



