352 B. F. KINGSBURY 



bodies or persistent epithelioid structures developed from this 

 complex is subject to a quite different interpretation, as has 

 been previously stated, and this will be considered again sub- 

 sequently. 



Recapitulation 



Thus the downward movement of material in the shiftings 

 as a result of unequal growth, usually summed up under the 

 descent of the heart, involves the branchial region as well. The 

 thyreoid follows the aortic trunk, but expanding laterally its 

 descent is arrested, as has been described. The third pouch 

 complex, from its peculiar position in relation to the forward 

 growth of the propharynx and to the truncus and aortic arches, 

 becomes peculiarl}^ involved, being drawn in ventrally, the 

 ventral prolongation (diverticulum) following by growth the 

 pericardium throughout the descent. The entire pouch com- 

 plex, however, moves down and becomes differentiated into a 

 head (embodying the parathyreoid III), body (thymus) and 

 intermediary portion, which in man becomes attenuated and 

 ruptures. Likewise and to a less degree, the more deeply located 

 caudal pharyngeal complex or Complex IV participates in the 

 movement and undergoes a comparable differentiation, the 

 initial complexity and the ultimate fusion with the thyreoid 

 differentiating it from the Complex III. The following diagrams 

 may crudely schematize the movement of these three pharyngeal 

 componants — thyroid. Complex III and Complex IV. In 

 schema C the arrows serve to indicate the general line of move- 

 ment for (a) the thyroid, (b) Complex III, and (c) Complex 

 IV, in horizontal and sagittal projection, while in figure D suc- 

 cessive positions for each of these three pharyngeal derivatives 

 are superimposed. Figure D is purely schematic, but figures 

 23 and 24 and the figures of plates 1 to 4 may be consulted in 

 comparison, as also the text figures 1 to 8. 



The descent movement is accompanied by rotation, most 

 obviously shown in the case of the parathyreoids, as already 

 described. As the ventral portion of Complex III is drawn into 

 the 'heart eddy,' a rolling or rotation upon its long axis gives to 



