DEVELOPMENT OF HUMAN PHARYNX 355 



ganglia seem to be derived from them (Landaere, v. Kupffer, 

 Froriep). In higher forms this is less clearly shown. The 

 intimate association of neural crest ganglion and placodal ecto- 

 derm, however, is clearly of marked significance. 



Epibranchial placode I — which, as in the territory of the 

 propharynx, lies outside the scope of the present study — ^has 

 in the 7.5 nmi. embryo apparently severed its connection with 

 the ganglion VII (geniculatum) with which it was doubtless 

 confluent at an earlier stage. In a 6.2 mm. cat embryo (Har- 

 vard Embryol. Coll. No. 380) placode and ganglion (N. VII) 

 are intimately associated. 



In contrast with the placode I, placodes II, III and IV main- 

 tain their close association with their corresponding nerves and 

 ganglia, and clearly play a part in the development of the ductus 

 branchialis II and cervical vesicle, as will subsequently appear. 

 In the 7.5 mm. embryo, placode II is a thickening of the ento- 

 derm lining, a depression behind the upper end of the second 

 branchial cleft, in intimate contact with the glossopharyngeal 

 nerve at the lower end of the ganglion (petrosum; fig. 25). Cor- 

 responding thickenings and depressions behind the upper ends 

 of the third and fourth clefts, as placodes III and IV, (figs. 27, 28) 

 are in close relation to the ganglion of the vagus nerve. With 

 the increase in thickness of the arches and the consequent re- 

 moval of the gangUon petrosum of the IX and ganglion nodosum 

 of the Xth nerve from a more superficial position, the placodal 

 ectoderm remains closely associated with the corresponding 

 ganglia, while there are produced the ectodermal insinkings 

 of the ductus branchiahs II and cervical vesicle respectively. 



In the 9.4 mm. embryo (fig. 19) ductus branchialis II has be- 

 come quite prolonged, while the cervical sinus has deepened and 

 narrowed, but both open to the exterior into the outer portion 

 of the cervical sinus by separate' apertures (fig. 10). By further 

 growth, in the 10 mm. embryo the cervical vesicle and ductus 

 branchialis II now communicate with the exterior through a 

 common tubular opening (figs. 15-17). Figure 17 in particular 

 shows the marked and rapidly acquired effect growth has pro- 

 duced in the wide separation of structures closely adjacent in 



