356 B. F. KINGSBURY 



the 7 mm. embryo. The ectoderm associated with the gang- 

 hon petrosum is now prolonged into a long tube extending to 

 the lower pole of the ganglion. The crest of the sinus cervi- 

 calis (representing the vagal placodal ectoderm) is imbedded in 

 the lower end of the ganglion nodosum, which has been removed 

 to reveal it. The connection of the Complex IV with the cer- 

 vical vesicle, close at 9.4 imn. (fig. 19) has now by growth and 

 descent become drawn out (fig. 17) to a slender tubular cord 

 (D. branchialis IV), soon to be ruptured, as in the 13 mm. em- 

 bryo, where on the left it is ruptured and no longer recognizable, 

 on the right, just ruptured (fig. 20). The connection of the 

 indrawn ectoderm, as the D. branchiahs II and cer\ical sinus 

 (through the ductus cervicahs) ^\'ith the superficial ectoderm, 

 becomes speedily severed and for some time (figs. 20, 22) the 

 upper portion of the ductus persists as a dmndling vesicle close 

 to the lower pole of the ganglion petrosum; while the cervical 

 vesicle remains in intimate contact with the ganghon nodosum, 

 where it gradually disappears, apparently without trace. 



By growth of the surrounding structures the sinus cervicalis 

 may thus be described as becoming divided into external^ and 

 internal portions, the latter again including the ductus branchiahs 

 II and the cervical sinus. These two structures obviously owe 

 their existence and form relations in part to the intimate con- 

 nection of the ganglia petrosum and nodosum respectively, 

 with the epibranchial ectoderm and the persistence of such 

 connection, under the expansive growth of the region. It is 

 likewise clear that the name, ductus branchialis II, is not appro- 

 priate, since it clearly owes its existence to the persistence of a 

 connection of nerve and epibranchial ectoderm. In its develop- 

 ment, it is true, the second cleft becomes in part incorporated, 

 so that the second pouch adjoins it in a characteristic manner 

 (fig. 15), but the 'duct' projects beyond it toward the ganglion 

 of the glossopharyngeal nerve (figs. 15, 17, 19). This conclu- 

 sion is further borne out by the fact that in a 9.2 mm. embryo 

 (Harvard Embryol. Coll. No. 734) this ductus branchialis opens 



" Sulcus cervicalis, or sulcus precervicalis, of Hammar. 



