CRANIAL GANGLIA OF AMBLYSTOMA 449 



the balancer fused with the dorsal extremity of the ventral 

 hyomandibular primordium. 



The epibranchial placode of VII still persists in contact with 

 the visceral sensory ganglion of VII where many mitotic figures 

 can be seen. The epibranchial placodes of IX and X still persist 

 and a few cells appear to be splitting" off to the visceral sensory 

 ganglia. 



The two primordia of the occipital group of lateral-line sense 

 organs have increased in size and their close contact to the IX 

 and X lateral-line ganglia gives them the appearance of contrib- 

 uting cells to those ganglia. The primordium of the dorsal 

 body line of sense organs now appears as an elongated placode 

 extending posterodorsally from the middle of the second somite 

 to the middle of the third somite, connected at its anterior end 

 by a slender lateral-line nerve to the posterior extremity of the 

 vagus lateral-line ganglion whose position is indicated by dotted 

 line. The midbody lateral-line placode is an elongated club- 

 shaped structure extending from the middle of the fourth somite 

 to the posterior border of the seventh somite. It is connected 

 from its anterior border by a slender lateral-line nerve to the 

 vagus lateral-line ganglion in common with the lateral-line nerve 

 of the dorsal body line. The ventral body line is represented by 

 a small ovoid placode which lies on the posterior border of the 

 fourth branchial arch. It is connected dorsally to the posterior 

 portion of the vagus lateral-line ganglion by a small lateral-line 

 nerve. 



Stage 36 



Neural crest. Since stage 35, there has been a more rapid 

 migration of the crest cells to the median border of the arches 

 where they accumulate in large numbers (fig. 32). As one studies 

 the sections farther ventrally, this becomes more apparent, and 

 it can be seen that the process is carried on at the expense of the 

 crest cells on the lateral, anterior, and posterior borders of the 

 arches where they lie in a thin layer, or where, in the case of the 

 lateral borders, they are occasionally entirely lacking. Within 

 the aggregation of crest cells on the median borders of the arches 



