338 F. L. LANDACRE 



of the origin of the lateralis Xth. The fact that there may be 

 lateral mass cells in the VHIth, however, does not affect the homo- 

 geneity of this system of ganglia, since the acustico-lateralis system 

 is based on anatomical and physiological characters. We have lat- 

 eralis ganglia, as in the case of the Vllth, derived solely from the 

 lateral mass, probably from the neural crest in other types, and 

 on the other hand lateralis ganglia, as in the case of the lateralis 

 Xth to be described later, derived solely from the postauditory 

 placode which is the posterior extension of the auditory vesicle. 

 The VIII may be intermediate between these two extremes, since 

 it possibly derives cells from both sources. The diversity in mode 

 of origin shown by acustico-lateralis ganglia emphasizes the fact 

 that the ultimate basis for the establishment of this system of 

 ganglia and nerves rests on anatomical characters and on the cen- 

 tral connections of this system in the brain and not on embryolo- 

 gical evidence, since some of its ganglia come directly from the 

 lateral mass like the general cutaneous ganglia, while others 

 arise secondarily, coming from the auditory vesicle or placodes, 

 and show a more specialized mode of origin. 



There is a great deal of variation in the extent to which these 

 three ganglionic masses (VHIth and two lateralis Vllth) become 

 separated from each other at any given stage. Sometimes the 

 dorso-lateral Vllth is free at one or both ends, while in other 

 embryos of the same age one or both ends may be incompletel}* 

 separated from the adjoining ganglia. There is also much varia- 

 tion in the relative lengths of the two divisions of the lateralis 

 Vllth, the dorso-lateralis Vllth sometimes extending far foward 

 on the lateral surface of the Gasserian. Up to the 86-hour stage 

 which I have plotted and in which the three divisions are isolated 

 (fig. 83) the variations strike one as representing different degrees 

 of isolation simply, and in this stage are in such a condition as 

 Herrick ('99) describes for the acustico-facial complex in Menidia. 

 After the 86-hour stage the ganglia seem to be in various stages of 

 assembling into the adult condition of Ameiurus in which they 

 are much more closely fused than in Menidia. In fact, the 86-hour 

 stage of Ameiurus seems to be in about the same condition as the 

 adult ganglia of Menidia. 



