THE CRANTAL GANGLIA IN AMEIURUS 325 



THE DIFFERENTIATION OF THE NEURAL PLATE 



Ameiurus presents a rather striking difference from the descrip- 

 tions usually given of the formation of the neural cord in its rela- 

 tion to the origin of the neural crest and dorso-lateral placodes. 



Balfour ('75) described the neural crest as growing out of the 

 neural cord but seems not to have worked stages sufficiently 

 early to determine its exact mode of origin. Marshall (77) gives 

 substantially the same description of its origin. Beard ('88), 

 however, describes it as arising in elasmobranches, teleosts, 

 amphibians, reptiles and birds as a thickening of the epidermis, 

 lying lateral to the neural plate and always distinguishable from 

 that structure before the neural tube is formed. There is sub- 

 stantial agreement among all the earlier descriptions of the cere- 

 bral ganglia in attributing to the neural crest a definite structure 

 related more or less closely to the dorsal portion of the neural 

 plate as it folds off to form the neural cord (Harrison '01, text- 

 figs. 1-5) . Few authors, so far as I am aware, have found any 

 close relation between the neural crest and dorso-lateral placodes 

 in their earliest stages. The neural crest ganglion is almost always 

 described as growing down ventrally from its point of origin and 

 coining into contact with the epidermis at the point of origin of 

 the dorso-lateral placode on a level with the notochord. Wilson 

 and Mattocks ('97, p. 659) do, however, describe the dorso-lateral 

 placode of the salmon as arising not by a thickening of the epi- 

 dermis but by the thinning out of the neural shield which leaves 

 the placode isolated, lying in a lateral position. In the salmon 

 the placode is at first located in the lateral portion of the neural 

 shield just as in Ameiurus. 



The early stages of Ameiurus differ from the usual descriptions 

 in that the neural crest and dorso-lateral placodes are not differen- 

 tiated from each other at first but appear as a large lateral thicken- 

 ing lying on either side of the neural plate. This I have desig- 

 nated as the lateral mass (fig. A.) It doubtless contains regions 

 comparable to the neural crest and certainly to the dorso-lateral 

 placodes of other authors; but since these are not recognizable 

 in the early stages and in fact the neural crest is never recognizable 



