318 J. A. BADERTSCHER 



thymus which is derived from the third pharyngeal pouch is 

 a comparatively long mid-ventrally directed epithelial tube before 

 the cervical sinus is fused to it. He also makes no mention of 

 the XII cranial nerve which plays an important part in modi- 

 fying the topographical relations of the anterior portion of the 

 thymus to surrounding structures. Basing his conclusions on 

 inaccurate observations, his views in regard to a mixed origin 

 of the thymus have now only an historical value. 



The first detailed study of the early development of the thymus 

 in the pig was made by Kastschenko ('87). He describes the 

 mesial portion of the sinus cervicalis, which he calls the 'vesicula 

 thymica,' as fusing with the anterior end of the epithehal anlage 

 of the thymus, which is derived from the third pharyngeal 

 pouch. In the shif tings of some of the structures in the neck, 

 that occur in young embryos during growth, the lateral portion 

 of the cervical vesicle is separated mechanically from its mesial 

 portion by the h3^oglossal nerve. The free lateral portion 

 of the cervical vesicle gives rise to the 'thymus superficialis' 

 which is necessarily of ectodermal origin. He claims that 

 the superficial thymus is not a constant structure, for, in a 

 30 mm. embryo that he examined, it was not present. The 

 anterior end of the thymus to which the mesial portion of sinus 

 cervicalis has fused, plus the parathyroid, that lies close to it, 

 he designates the thymus head; while the large remaining por- 

 tion of the thymus which is of a purely entodermal origin, plus 

 the thymus head, he calls the 'thymus profunda.' The largest 

 embryo examined by him was 82 mm. in length. 



Zotterman ('11) also made a detailed study of the morpho- 

 genesis of the thymus of the pig. Her conclusions are in accord 

 with those of Kastschenko with the exception that in about 

 one-half of the specimens examined the superficial thymus was 

 connected with the thymus head by a cord of cells that looped 

 over the hypoglossal nerve. The superficial thymus was found 

 in all the specimens examined and in the largest (105 mm.) 

 investigated aU the features common to the thymus (cortex, 

 medulla, Hassall's corpuscles, etc.) were present. 



Fox ('08) agrees with Kastschenko that the superficial thymus 



