348 B. F. KINGSBURY 



posed largely of parathyreoid III moves down, for a time, 

 although more slowly, mitil the parathyreoid III has consider- 

 ably passed the parathyreoid IV. (b) The intermediary 

 epithelial cord (thymic cord of Hammar) joining the caudal 

 portion (thymus) to the cephalic portion (parathyreoid III) 

 becomes attenuated and ruptures (in embryos of about 35 mm. ± 

 length). The parathyreoid III, as a result of small growth 

 shiftings and largely through the expansion of the thyreoid, 

 comes to occupy its adult position as the inferior parathyreoid. 



The ventral pocket of the third pouch is from the beginning 

 quite close to the pericardium and retains this intimate associa- 

 tion throughout the descent (figs. 23-24) so that the statement 

 of Hammar ('11, p. 217) that "after entering the thoracic cavity 

 the thymus comes into relation with . . . the pericardium" 

 hardly, I think, conveys the correct impression of the topographi- 

 cal relation of thymus to pericardium in the course of develop- 

 ment. The small amount of mesenchyme between the branchial 

 epithelium and the pericardial mesotheUum does, in fact, be- 

 become thinned out, doubtless due in part to the expansive growth 

 of both structures. 



Critical and careful examination of the series of sections of 

 embrj^os of progressively advanced stages has convinced me that 

 the mesenchyme surrounding the third pouch participates in 

 the down-growth accompanying the heart's descent, so that it 

 is not only the branchial epithelium but the material of the 

 region as a whole that suffers the growth displacement which 

 is so characteristic of it. What significance this may have in 

 the later thymic transformation can only be inferred (vide 

 subseq.). The parathyreoid III and the thymus, which remain 

 as 'derivatives' of the transformation of the Complex III, will 

 be discussed subsequently. 



Com'plex IV 



The transformations undergone by the fourth pouch and 

 associated epithehum comprised under the name of 'caudal 

 pharyngeal complex,' or Complex IV, have already been briefly 



