DEVELOPMENT OF HUMAN PHARYNX 371 



to an entirely similar point of view regarding the homologiza- 

 tion of tonsils in lower and higher forms (Kingsbury '12). 



Parathyreoids 



The characteristic number of four parathyreoids III and IV 

 on the two sides appears in man to be quite constantly present. 

 In the series of 60 embryos examined, many of them attaining 

 beyond the period of differentiation of these structiu-es from the 

 other portions of the pharyngeal epithelium (ca 20 mm.), in but 

 two instances were accessory parathyreoids noted. In the 

 40 mm. embryo, there were four parathyreoids on the right side, 

 the two extra bodies, from their position, being undoubtedly 

 derived from both III and IV. In the 18 mm. embryo there was 

 a small accessory parathyroid III on the left. But little has been 

 added to the known mode of development of these structures 

 in man, as set forth by Grosser ('11 b) and more recently by 

 Hammar ('11). It has been possible, however, to follow them a 

 little farther back in development, to emphasize their funda- 

 mental morphological relations and to offer a suggestion as to 

 their morphological significance. 



They develop, as already described (Hammar '11), as a thick- 

 ening of the branchial epitheUum in the dorsal portion of the 

 third and fourth branchial pouches respectively, upon their 

 lateral, and to a slight extent (morphologically), cephalic aspects. 

 The thickening is attended by and at first in large part at least 

 due to a peculiar loosening or reticulation of the epithelium. 

 Parathyreoid III was first clearly recognizable (fig. 18; also 

 fig. 17) in the 7.5 mm. embryo, while the parathyreoids IV were 

 not yet distinctly recognizable. In the subsequent growth their 

 characteristic appearance is maintained. The thickening of 

 the epitheUum continues and with the (typical) degeneration and 

 disappearance of the contiguous branchial epithelium they re- 

 main as subspherical-oval bodies in the adjacent mesenchyme. 

 How far their increase in size in the early period of growth is 

 due to extension of the parathyreoid-forming process to the 

 adjacent epithelium, and to what extent it is due to only enlarge- 



