350 B. F. KINGSBURY 



from it (figs. 20, 21, 22). The parathyreoid III thus rotates 

 from a lateral position upon the dorsal aspect of the pouch to 

 a more ventral one (Hammar '11, p. 207). The dorsal position 

 of the Complex III shares thus the ventral movement so pro- 

 nounced in the growth of its ventral moiety. The Complex 

 IV — which, as seen, is more deeply located — comes less under 

 the influence' of the descent and grows and shifts caudally 

 more slowly and to a less degree. The ventral diverticulum 

 (IV), like the ventral pocket of the third pouch, at first as it 

 lies between the fourth branchial aortic arch and the pulmonary 

 arch is well marked and directed like that of the third pouch 

 toward the bifurcation of the aortic trunk (figs. 12, 13, 3). It 

 does not, however, like the third pouch, follow the caudal move- 

 ment of the truncus but is soon withdrawn into the complex and 

 is thenceforward indistinguishable. 



A caudal movement of the complex as a whole also takes place, 

 although more slowly than the fourth aortic arch. The broad 

 communication of the Complex IV with the pharynx, on the 

 one hand (ductus pharyngeo-branchialis IV), and its connection 

 with the cervical sinus (ductus branchialis IV), on the other, 

 are speedily stretched to slender cords or tubes and ruptured, 

 the latter first in embryos of 10 to 13 mm. length, (figs. 15, 17, 

 20), the latter somewhat later, in embryos of ca. 14 mm. length 

 (figs, is, 21, 22). With the rupture of the ductus branchialis 

 IV, in the 10 nun. embryo (fig. 17) already thinned and bowed, 

 the fourth aortic arch passes it by in the descent upon the outer 

 side. As this occurs the complex follows it by a rotation where- 

 by the parathyreoid IV moves from a position upon the lateral 

 aspect (fig. 15) in the dorsal portion (morphologically) of the 

 complex to a dorsal one (figs. 20-21), rotating thus roughly 

 through ninety degrees, as does the parathyreoid III, but in an 

 opposite direction. 



After the rupture of the ductus pharyngeo-branchialis IV, 

 a somewhat more rapid movement of the caudal portion of the 

 body of the complex causes the appearance of a more attenuated 

 portion (neck) to appear, whereby it is joined to a head con- 

 sisting mainly of parathyreoid IV (fig. 8). Subsequent rupture 



