THE LATER DEVELOPMENT OF THE FROG 139 



eo 



FIG. 45. Portions of sections through the head of the frog (R. fused), illus- 

 trating the formation of the placodes and the history of the crest ganglia. After 

 Brachet. A. Transverse section through the trigeminal ganglion of an embryo 

 of 3 mm. B. Transverse section through the acustico-facialis ganglion of an 

 embryo with three or four pairs of mesodermal somites. C. Transverse section 

 through the facial ganglion and auditory placode of an embryo of 2.8 mm. 

 ei, inner or nervous layer of ectoderm; en, endoderm; eo, outer layer of ectoderm; 

 m, mesoderm; mpd, definitive medullary plate; n, nerve cord; pa, auditory pla- 

 code; pf, facial placode; ptg, trigeminal placode; r, spinal prolongation of ganglion; 

 tg, trigeminal ganglion. 



