DEVELOPMENT OF HUMAN PHARYNX 349 



discussed by me (Kingsbury '14 b) from a somewhat different 

 point of view, so that it is only necessary to supplement some- 

 what what was then said and with more particular reference to 

 the growth transformations of the region as a whole. 



The Complex IV from the method of its development occupies 

 a position not only morphologically caudad to the third pouch, 

 but also from its later development and growth is almost imedi- 

 ately medial and dorsal to it, the fourth arch intervening (figs. 

 1-3, 12, 13). It therefore is not so intimately involved in the 

 descent of the heart as is the third pouch complex. Growth 

 continuing after the establishment of the fourth pouch, there is 

 formed the so-called ultimobranchial body, which, however, 

 I do not find extending to or toward the ectoderm, and hence 

 hardly to be given the value of a fifth or of a sixth pouch. The 

 question of the significance of this body has already been dis- 

 cussed in the earlier paper. The so-called ultimobranchial body 

 lies next the laryngeal mesoderm and between it and the fourth 

 (fifth) arch. At this stage (5-7 mm.) the Complex IV consists 

 of the so-called thymus IV- — the ventral diverticulum of the 

 pouch, the portion making contact with the ectoderm and the 

 so-called ultimobranchial body. The form and position of the 

 complex is shown, not quite satisfactorily, for the 5 mm. embryo 

 in figure 12, while figures 13 and 14 show the corresponding 

 structures in the 7.5 mm. embryo. Figure 3 is a section through 

 the ventral diverticulum IV. These regions of the Complex IV 

 speedily become lost in the transformations which it undergoes, 

 and a fourth portion is differentiated^ — the parathyreoid IV — 

 already well indicated at 8.3 mm. (figure 30) although not recog- 

 nizable in the 7.5 mm. embryo. 



. In order to understand the part that the descent of the heart 

 plays in these transformations and growth shiftings, it is essential 

 to keep in mind the relative movements of the third and fourth 

 arches. The third arch has become the common carotid artery, 

 and in its 'migration' out upon the fourth arch it becomes rela- 

 tively more dorsally located. The head of the third pouch 

 complex, at first morphologically caudad to it, moves thus 

 around to its ventral side, becoming soon more widely separated 



