CRANIAL GANGLIA OF AMBLYSTOMA 441 



(fig. 5). In the lower portion of the hyomandibular cleft, on a 

 level with the ventral border of the optic vesicle, appears an 

 elongated ectodermal thickening, where later primordia of the 

 ventral hyomandibular and mandibular groups of lateral-line 

 sense organs appear. 



The epibranchial placode of VII can be seen as a small ecto- 

 dermal thickening somewhat deeper than in stage 26-27, lying 

 on the upper anterior border of the hyoid arch just below the 

 lateral-line ganglion placode of VII (fig. 23). The epibranchial 

 placode of IX appears in approximately the same position as in 

 the previous stage and is preparing to give off a mass of cells 

 (fig. 19). The vagus lateral-line placode has begun to give off 

 a large mass of cells which comes in contact with a thin layer of 

 the neural crest. Above the dorsal extremity of the first bran- 

 chial cleft is the small epibranchial placode which is a visceral 

 sensory placode of the vagus. In the region below the lateral- 

 line ganglion placode of the vagus the ectoderm is somewhat 

 thickened and represents the posterior portion of the visceral 

 sensory placode of the vagus. In the intersegment of the first 

 and second somites, the prominent ectodermal thickening appears 

 somewhat larger than in stage 26-27, 



Stage 33 



Neural crest. The rapidly diminishing area of neural crest in 

 the region over the dorsal portion of the eye is seen in this stage 

 (fig. 6). The crest cells have passed farther behind the nasal 

 placode on its posterior border where they join, along the ventral 

 border of the optic vesicle, those which surround the maxillary 

 process. The crest cells over the dorsal border of the hyoid 

 arch are continuous with a broad area of crest cells which lie 

 upon the anterior and ventral borders of the auditor}^ vesicle. 

 The mesoderm of the hyoid and first branchial arches is covered 

 by crest cells which have now extended to the ventral borders of 

 these arches. The crest cells lying above the remainder of the 

 branchial region have diminished in number and now lie in a 

 thin layer. The neural crest can no longer be found over the 

 dorsal border of the auditory vesicle. The broad sheet of crest 



