310 OUTLINES OF CHORDATE DEVELOPMENT 



placode, just above the second and third visceral pouches. 

 These placodes are vestigial organs in terrestrial vertebrates, 

 and this connection, as well as the existence of the placodes 

 themselves, is of very brief duration. 



The ganglia of the VII and VIII nerves arise in common, 

 from an enlarged part of the crest lying immediately in front 

 of the rudiment of the ear. This cell-group, the acustico- 

 facialis ganglion (Fig. 114), connects with the brain in the 

 region of the third neuromere of the myelencephalon. It also 

 connects (about the forty-eighth hour) with a well-marked pla- 



l&is. f.C/i 



I/. 



Lens 



FIG. 125. Transverse section, passing through the eyes and heart, of an 

 embryo with about thirty-five pairs of somites (about seventy-two hours). 

 From Lillie (Development of the Chick). Am., Amnion; Ao., dorsal aorta; 

 Atr., auricle; B.A., bulbus arteriosus; Ch.Fis., choroid fissure; Chor., chorion; 

 D.C., ductus Cuvieri; Dienc., diencephalon; Lg., rudiment of lung; P. C., pericar- 

 dial cavity; p.Ch., posterior chamber; pl.gr., pleural groove; V.c., posterior car- 

 dinal vein; Y.S., yolk-sac. 



code on the first visceral pouch. The ganglion of the V nerve, 

 trigeminal ganglion, is the most anterior of the series (Fig. 130). 

 It connects with the first neuromere of the myelencephalon and 

 sends outgrowths into the regions of both upper and lower 

 jaws, as the rudiments of certain branches of the V nerve. A 

 part of the mesenchyme of the mandibular and hyoid arches 

 is also formed from the cells of these rudiments, and especially 

 from the neural crest anterior to the V ganglion. During an 

 early stage in their formation from the crests, the rudiments 

 of the V and VII ganglia connect with the surface ectoderm 



