DEVELOPMENT OF HUMAN PHARYNX 343 



the branchial pharynx attains its most marked development. 

 The transformations that speedily succeed are marked by the 

 growth shiftings, whose most striking and central features center 

 around about the descent of the heart^ the influence of which 

 on the subsequent development of the pharynx is striking. 



Thyreoid 



In the 3 mm. embryo — the youngest of the series studied — 

 the thyreoid gland is present as a median outpocketing, still 

 broadly in communication with the caudal limb of the first bran- 

 chial pouch and in immediate contact with the truncus aorticus. 

 The first (mandibular) aortic arch is disappearing, the second 

 and third continuous to the dorsal aorta, while the fourth (ap- 

 parently) has not yet become completely developed. In the 5 

 mm. embryo the thyreoid is somewhat elongated transversely 

 and is close to the aortic trunk bifurcation, which has 'moved' 

 caudally somewhat in the process of growth. The bifurcation 

 has now a somewhat different morphological value as compared 

 with the earher stage. The second aortic arch has 'moved out' 

 along the third arch, while the first arch has apparently 'moved 

 out' along the second, the first arch seemingly having lost its 

 connection with the dorsal aorta. As at some points these ves- 

 sels were collapsed, it was somewhat difficult to determine their 

 exact extent. The condition appears very similar to that in 

 the His embryo II (figure 120). The apparent migrations of 

 the first and second arches are undoubtedly correlated with 

 the growth of the region as a whole. The anterior portion of 

 the pharynx, including the first and second arches, expands in 

 a 'cephalic' direction, and the growth of these bilateral moieties 

 carries the arches with them, forming thus the so-called ventral 

 aortae. The median structures (thyreoid, truncus, heart, and 

 the pericardial cavity) do not share in the forward growth, and 

 tljere is thus instituted the 'descent of the heart.' The neck 

 and thoracic wall, in completely burying the heart and blood 



- In the following pages the term 'descent of the heart' is, for convenience, 

 frequently used and represents a growth shifting that has, of course, many 

 sides. It is used without the intention of giving this side of it undue emphasis. 



