DEVELOPMENT OF HUMAN PHARYNX 373 



fore, merely as a suggestion that has several sides: (1) that 

 possibly herein lies the significance of the fact that the third 

 and fourth pouches are the only ones that develop parathyreoids, 

 the third and fourth arches alone persisting; (2) the parathyreoids 

 may thus more closely represent the primary branchial epithelium 

 and more truly deserve the name of branchial remnants than 

 other branchial derivatives; (3) Maurer's contention that the 

 carotid body (in amphibia, and other forms?) represents an 

 epithelial body II (parathyreoid II) may herein find support 

 and confirmation. In any event, a reexamination of the origin 

 of the epithehal bodies and their vascular relations in amphibia, 

 as well as those forms in which the development of an epithelial 

 body (parathyreoid) II has been described, is called for. 



However interpreted as to its primary significance, the asso- 

 ciation of parathyreoid and corresponding arch persists for some 

 time (fig. 20), and the parathyreoid follows the growth shif tings 

 of the corresponding blood vessel, the parathyroid IV rotating 

 toward the dorsal side, the parathyreoid III toward the ventral 

 side of the corresponding pouch complex, as above described. 

 Ultimately, of course, the vascular arch moves away from the 

 parathyreoid, which remains stranded in the midst of the mesen- 

 chyme by the degeneration of the associated branchial epithelium. 



Thyreoid 



This derivative has received but little attention in this study, 

 aside from its form changes already considered. The separation 

 from the pharynx is early and complete, so that a thyreoglossal 

 duct — in no sense a duct, any more than are the other so-called 

 pharyngeal ducts — does not at any time normally exist, or has 

 only a brief existence. The point of separation from the pharynx, 

 while always recognizable from the first as a shallow depression, 

 is only deepened subsequently (13 to 14.5 mm.) apparently 

 mainly by growth of the adjacent mesoderm. The beginnings 

 of a pyramidal lobe, clearly a growth of cells in the line of descent, 

 was first found at 14.5 mm. The cavity of the thyroid is speedily 

 lost (5 mm.) and the period of expansion begun by the peri- 



