DEVELOPMENT OF HUMAN PHARYNX 351 



and disappearance of the neck separates the two portions, leav- 

 ing the parathyreoid — at this time near the upper pole of the 

 thyreoid lobe — upon its dorsal side, in the interval between 

 esophagus and trachea, therefore near the position which it 

 typically occupies as the superior parathyreoid. 



At the same time that the Complex IV is more slowly descend- 

 ing in a more dorsal position and more deeply placed, and the 

 Complex III is descending by growth more rapidly in a more 

 ventral and superficial position (figs. 20, 21) the thyreoid is 

 rapidly extending in the intermediate interval between these 

 materials, following the carotid artery as it shifts relatively 

 more dorsally (figs. 4, 6,y7, 8). The body of the complex is thus 

 enveloped by the expanding thyreoid lobe, is brought in contact 

 with its medial dorsal edge and fuses with it (fig. 8). Nothing 

 further relative to its fate can he added to the discussion already 

 published. 



Considerable variability characterizes the morphological trans- 

 formations of the Complex IV, which is particulary apparent 

 in the models from the 13.0 mm., 14.5 mm., and 18.2 mm. em- 

 bryos, in which the two structures upon the two sides are mark- 

 edly different. Differences between the two sides are evident 

 in most of the embryos of about this size or larger. In the 18.2 

 mm. and 31.0 mm. (1) specimens, tubular epithelial prolonga- 

 tions from the complex occur in the mesenchyme upon the dorso- 

 lateral aspect of the thyreoid, while in the 32 mm. embryo a 

 thymus IV is present outside the thyreoid. These variations 

 in the form of the complex clearly indicate differences in the 

 conditions that determine its growth and in which altered en- 

 vironment in undoubtedly a factor. 



Aside from the early differentiated area that becomes the 

 parathyreoid IV, it is impossible to distinguish and follow, in 

 the transformations which the complex undergoes, any regions 

 of specific structural value, such as a thymus IV, or a corpus 

 ultimobranchiale as a definite organ of however 'vestigial' a 

 character, and it may be asserted that the evidence of the 

 existence of such structures as intrinsically branchiomeric organs 

 is exceedingly meager. The occasional occurrence of thymus 



