306 The Ohio Journal of Science [Vol. XX, No. 8, 



later, and give rise, not to ganglia, but to specific lateral line 

 organs. If, as in the urodele, the lateral line primordia arise 

 early, the relations are not so easy to understand as in Lepi- 

 dosteus, where there is a considerable time interval separating 

 the appearances of the two structures. 



From this series of placodes we get the lateral line ganglia 

 of the Vllth, Vlllth, IXth and Xth nerves, and, with the 

 exception of the Vlllth, the behavior shows a fair degree of 

 uniformity. The following variations should be noted: 



(a) If the neural crest remains in the lateral ectoderm, as 

 described in the second type of behavior, illustrated by 

 Ameiurus, a large lateral mass is formed containing both the 

 neural crest and the dorso-lateral placode combined in such a 

 manner that it is very difficult at first to differentiate the two 

 components. This condition in Ameiurus, however, is to be 

 interpreted in the light of those forms which present a distinct 

 neural crest and distinct dorso-lateral placode. 



(b) Sometimes, as indicated above, it may be difficult to 

 differentiate a dorso-lateral placode which forms ganglia from 

 lateral line placodes which form lateral line organs, but I believe 

 these structures to be morphologically distinct, and if any given 

 case they are not well separated by a time interval hi appear- 

 ance, they should still be interpreted in the light of those forms 

 in which there is a well defined time interval between the two 

 sets of thickenings. 



(c) Next, attention should be called to the mode of origin 

 of the auditory ganglion. Authorities are almost equally 

 divided as to whether it rises, on the one hand, from the auditory 

 vesicle, or on the other, from the neural crest. If it rises from 

 the neural crest, and is not included in (a) above as illustrated 

 by Ameiurus, it is an exception to the behavior of the lateral 

 line ganglia, and I have no suggestion to make as to the interpre- 

 tation of this condition. However, in the types I have studied 

 it seems to come from the auditory vesicle. The precocious 

 appearance of the auditory vesicle seems to have incorporated 

 the dorso-lateral placode at this level and forced it to appear, 

 not from the smooth ectoderm, but from the infolded vesicle. 

 The last word, however, has not been said concerning the origin 

 of the auditory ganglia, by any means. Streeter has shown that 

 in mammals it is distinctly double in form, and there are 

 indications, both in Ameiurus and in the urodeles, that this 



