332 J. A. BADERTSCHER 



A microscopical examination was made of the superficial thy- 

 mus in various developmental stages, including that of two full- 

 term embryos. It was found that the histogenetic processes of 

 this segment kept pace with those in the segments of the thymus 

 which have a purely entodermal origin. 



CONCLUSIONS 



The thymus of the pig has an ectodermal-entodermal origin. 

 The respective origin of each segment is as follows: 



1. The superficial thymus, which is a derivative of the cervical 

 vesicle, has a purely ectodermal origin. It is a constant struc- 

 ture and, therefore, forms an integral part of the organ. 



2. The connecting band is also a derivative of the cervical 

 vesicle and has, therefore, a purely ectodermal origin. In the 

 majority of embryos it persists to birth but may be absent 

 either on one or on both sides. 



3. The thymus head, in which is lodged the parathyroid III, 

 is formed by a fusion of a portion of the cervical vesicle to the 

 anterior end of the epithelial diverticulum derived from the third 

 pharyngeal pouch. It has, therefore, an ectodermal-entodermal 

 origin. 



4. The intermediary and cervico- thoracic cords, and the mid- 

 cervical and thoracic segments are derived wholly from the epi- 

 thelial diverticulum of the third pharyngeal pouch and have, 

 therefore, a purely entodermal origin. 



I wish to thank Dr. B. P. Kingsbury for the aid given me in 

 this work. I am also indebted to Dr. David Marine, of the 

 Western Reserve University, for sending me many formalin- 

 preserved embryos of various sizes from which most of the draw- 

 ings of the exposed thymus were made. 



