DEVELOPMENT OF HUMAN PHARYNX 



353 



it a slight spiral twist (18.2 mm.) which is rendered obvious by 

 the growth furrows in the beginning expansion, as well as by the 

 shifting of the thicker epithehum of the tube from a dorso- 

 lateral position higher, to a ventrolateral position lower down. 

 The latter of these features has been commented on by Ham- 

 mar ('11). 



The prevalent asymmetry in the adult relations seems clearly 

 due- to a corresponding asymmetry in the descent. Thus the 

 right parathyreoid III is usually lower than the left, while the 



Schema C-D 



pyramidal lobe of the thyreoid, when present, is in the majority 

 of cases asymmetrically placed, to the left. Inasmuch as the 

 pyramidal lobe unquestionably, I believe, marks the line of de- 

 scent of the thyreoid, both its more usual excentric position, as 

 well as the generally lower position, of the right parathyreoid 

 III indicates a more extensive descent tendency upon the right 

 side, probably associated with the asymmetry in the aortic arches 

 (IV), or is due to the asymmetrical position of the common caro- 

 tid arteries upon the right and left sides, associated therewith. 

 Decisive proof of this would be difficult to secure. 



