CRANIAL GANGLIA OF AMBLYSTOMA 491 



organs appeared, correspondingly small lateral-line ganglia were 

 present which innervated those sense organs. No evidence can 

 be obtained that crest cells contribute to the formation of lateral- 

 line ganglia. The morphological studies are misleading in this 

 respect, for in many cases the close arrangement and contact of 

 the early crest cells and placodes make an interpretation of the 

 exact contribution of the two kinds of cells difficult to understand. 



In the study of the normal embryos the epibranchial placodes 

 of VII, IX, and X could be located as early as stage 26-27 (fig. 4) 

 and their contributions to gangUa could be followed up to stage 

 36. The observations of these placodes agree in many respects 

 with Coghill as to the placodal relation and contribution to the 

 visceral ganglia. The removal of epibranchial placodes was 

 found to be accompanied by a distinct lack of gustatory fibers in 

 VII, IX, and X with no apparent disturbance to the general 

 visceral system. In this respect the experimental results agree 

 with Landacre's explanation of the function of the epibranchial 

 placodes in Lepidosteus, viz., that they give rise to special vis- 

 ceral ganglia of VII, IX, and X. 



Goette ('14) expresses the belief that the epibranchial and 

 lateral-line placodes form, with the crest cells, a syncytial mass 

 of cells out of which ganglia and nerves are formed which later 

 join themselves up with the brain. Studies of experimental 

 and normal amblystoma embryos show that certain definite 

 portions derived from placodes and crest cells, although they 

 mingle with each other, maintain their identity and are not to be 

 considered a syncytial mass at any time. 



It has also been shown in Amblystoma that the neural crest 

 originates from the dorsal portions of the contiguous surfaces of 

 the neural folds at the time of the closure (fig. 13). These crest 

 cells were followed by means of their difference in pigment and 

 by the presence of fine yolk granules in their cytoplasm as they 

 migrate ventrally over the mesoderm of the arches, always re- 

 maining separate from the ectoderm. The wandering mass of 

 ectoderm is of crest-cell origin only and does not in Amblystoma 

 receive any contribution from the ectoderm on the lateral sur- 

 faces of the head. Piatt's descriptions of Necturus show clearly 



