CRANIAL GANGLIA OF AMBLYSTOMA 



447 



near the dorsal extremity of the fourth branchial arch. This is 

 the primordium of the ventral body line of sense organs. It is 

 connected by a few placodal cells from its dorsal border to the 

 vagus lateral-line ganglion. 



^brcc 



32 



Fig. 31 Frontal section, showing at stage 35 the IX lateral-line ganglion 

 and placode anterior to epibranchial placode of X. Farther posteriorly is mid- 

 body-line primordium. X 37. 



Fig. 32 Frontal section, showing at stage 36 aggregation of crest cells on 

 median surfaces of mesoderm of hyoid, first and second branchial arches. 

 X 37. 



It is at this stage in the developing embryo that all of the pri- 

 mordia of the lateral-line groups of sense organs are laid down in 

 visible form in the ectoderm. 



