CKANIAL GANGLIA OF AMBLYSTOMA 451 



As one studies the ventral portions of the mandibular and maxil- 

 lary processes, it is evident that there is an abundance of crest 

 cells, especially around the former (fig. 33). 



Placodes. The supra-orbital primordium now extends as far 

 ventrally as the dorsal border of the olfactory organ (fig. 9), 

 while the infra-orbital descends to the posterior border of the 

 olfactory organ. The hyomandibular primordium has now joined 

 the dorsal extremity of the ventral hyomandibular primordium 

 posterior to the balancer. The mandibular primordium extends 

 anteroventrally, then mesially to the region of the lower jaw. 

 The ventral hyomandibular primordium extends farther ven- 

 trally, then mesially where it begins to take an anterior direction 

 upon the ventral surface of the lower jaw. Posterior to the 

 auditory vesicle, four separate placodes now mark the positions 

 of the primordia of the occipital group of lateral-line sense organs. 

 The two anterior placodes of this group lie close to the posterior 

 border of the ear, while the two posterior lie above the base of 

 the third external gill; the more ventral of the latter is much 

 elongated in the anteroposterior direction. 



The dorsal body-line primordium has extended .as far as the 

 seventh somite, followed by a thin lateral-hne nerve upon which 

 the anlagen of two sense organs have appeared. The primordium 

 of the midbody line is a very much elongated placode which 

 extends over about seven and one-half somites as far as the 

 sixteenth somite. Along its lateral-line nerve appear the anlagen 

 of five sense organs. The ventral body line can be seen descend- 

 ing behind the external gills anterior to the limb region connected 

 by a slender lateral-line nerve. 



The epibranchial placode of VII still has a slight contact with 

 the distal end of the visceral portion of VH. No other epibran- 

 chial placodes are found to be giving off cells to the ganglia at 

 this stage. 



Stage 37-38 



Neural crest. The aggregations of crest cells along the median 

 borders of the visceral arches now show a more compact arrange- 

 ment. So dense is the tissue that it appears to be composed 



