THE LATER DEVELOPMENT OF THE FROG 137 



ment of the acustico-facialis ganglion (VIII and VII nerves), 

 and the posterior as the rudiment of the ganglion of the glosso- 



FIG. 44. Sections through young frog embryos (R. fused), illustrating the 

 development of the crest ganglia and placodes. After Brachet. A. Transverse 

 section through the neural plate of an embryo before elongation begins. B. 

 Sagittal section, to one side of the mid-line, through an embryo of the same age 

 as A. (This is also the stage of Fig. 32, G.) C. Sagittal section, to one side of 

 the mid-line, through an embryo just beginning to elongate. D. Transverse 

 section through, an embryo slightly older than that of A and B. E. Frontal 

 section through an embryo with three or four pairs of mesodermal somites. 

 F, G, H. Three transverse sections through an embryo just beginning to elongate 

 (same age as C), showing the trigeminal, acustico-facial and glossopharyngeal- 

 vagus crest ganglia, a/, Acustico-facialis ganglion; c, notochord; en, endoderm; 

 g, gut cavity; gl, glossopharyngeal ganglion; gv, glossopharyngeal-vagus ganglion; 

 I, liver diverticulum ; m, mesoderm; mp, primitive medullary plate; mpd, defini- 

 tive medullary plate; nc, neural crest; s, mesodermal somites; tg, trigeminal 

 ganglion; va, vagus (pneumogastric) ganglion. 



pharyngeal (IX) and vagus (X) nerves. The delamination of 

 these rudiments from the medullary plate is not quite complete, 



