DEVELOPMENT OF HUMAN PHARYNX 345 



material, and of the larynx particularly, soon removes the thy- 

 reoid from the intimate contact with the carotid arteries (18.2 

 mm.). Its adult morphology is then speedily assumed. Dia- 

 gram 3 of schema B may illustrate the third stage in its growth 

 transformation, based on the conditions in the 13. mm embryo 

 (fig. 20), and comparable with the well-known figures of Verdun 

 C98,pl.l). 



Complex III 



The above paragraphs clearly show the influence of the un- 

 equal growth and growth movements summed up as the descent 

 of the heart on the development of the thyroid, which from its 

 first appearance follows the aortic bifurcation. The third 

 branchial pouch is also drawn into the 'growth eddy' caused by 

 the descent of the heart. 



As has just been said, the branchial pouches are directed 

 toward the aortic bifurcation (figs. 12-13), this arrangement 

 being clearly due to the arrangement of the branchial arches. 

 From their mode of appearance — each pouch following in develop- 

 ment the one cephalad of it and therefore lagging behind it in 

 extent and size — the more cephalic pouches and arches tend to 

 overlap those more caudally located unless the relations are 

 disturbed by unequal growth. This arrangement appears in 

 figures 12, 13, and 1, 2. In the case of the first two pouches 

 this relation is inappreciable, or speedily lost. In the growth 

 of the head the propharynx including these pouches is carried 

 forward away from the mid-ventral or cardiac region. The 

 third pouch is thus not only morphologically cephalad of, 

 but also laterad to, the fourth pouch. It also occupies a posi- 

 tion of peculiar interest in the series of branchial pouches in its 

 relation between the third and fourth arches, it is at first (mor- 

 phologically) caudad to the common carotid artery. As the aortic 

 truncus and the thyreoid descend the third pouch soon becomes 

 lateral to these structures as they move by, and to the com- 

 mon carotid. This position is shown in figures 4, 7, 8, 15 and 20. 



As the truncus descends the third pouch follows it in its growth 

 movement. Drawn, so to speak, into the heart eddy, growing 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 18, NO. 3 



