CRANIAL GANGLIA OF AMBLYSTOMA 437 



mandibular arch is covered by crest cells to a level below the 

 ventral border of the eye. Sections show that the crest cells are 

 beginning to wrap around the mesoderm of the arch — a condition 

 which can be seen more clearly in stage 30. The hyoid and first 

 branchial crest-cell groups have extended farther ventrally to 

 about the same level. The hyoid group (fig. 17) is quite thick 

 and packed tightly against the arch. The first branchial group 

 is more slender and curves slightly toward the hyoid arch. The 

 remaining portion of the branchial mesoderm is being covered 

 by a band of the neural crest which is slightly constricted at its 

 union with the neural crest above. The posterior extension of 

 the neural crest is diminished as its cells migrate toward the gill 

 region, the extremity reaching only to a point above the posterior 

 border of the second somite. It is from this posterior portion 

 above the first three somites that the crest cells of the second, 

 third, and fourth branchial arches have their source. 



Placodes. Over the dorsal border of the eye the ophthalmic 

 placode is a long narrow thickening which gives the appearance of 

 having a broad contact with the loose neural crest against which 

 it lies (fig. 18). The triangular placode anterior to the auditory 

 placode approaches more closely the dorsal border of the crescent- 

 shaped thickening above the hyomandibular cleft (fig. 4). The 

 posterior margin of the placode of the lateral-line ganglion of VII 

 is gi\4ng ofT a large number of cells which approach the neural 

 crest lying along the anterior border of the auditory placode. 

 The epibranchial placode of VII is situated immediately below 

 and anterior to this cell proliferation as a very slight ectodermal 

 thickening lying close to the lower border of the lateral-line 

 placode. The epibranchial placode of IX lies in a level some- 

 what dorsal to its position in stage 25. On its inner surface loose 

 cells containing mitotic figures lie closely against the crest cells 

 descending on the anterodorsal margin of the first branchial arch, 

 indicating that placodal cells are being given off to form a gan- 

 glion (fig. 17). Posterior to the placode which lies in the upper 

 extremity of the first branchial cleft a somewhat elongated area 

 of the ectoderm is slightly thickened. It is very probable, judg- 

 ing from the study of older stages, that these placodes are the 



