THE CRANIAL GANGLIA IN AMEIURUS 361 



trace the genesis of the sensory lines. Miss Beckwith shows that 

 the lines from which the lateral line organs appear are entirely 

 distinct from the auditory vesicle and that the supra-orbital, 

 sub-orbital and mandibular sensory ridges arise separately. Locy 

 states that he agrees with Wilson but gives no description of the 

 origin of the specific lateral line organs. It will serve to clear 

 the ground for future work if it is remembered that there are two 

 distinct problems here; first, do the pre- and postauditory lateral 

 line primordia grow out from the auditory vesicle or its exten- 

 sions the pre- and postauditory placodes, and, second, do the lateral 

 line organs appear individually or do they differentiate from a 

 common primordium? 



Admitting that the lateral line organs do differentiate in some 

 types from common sensory ridges and in other types as indivi- 

 dual organs, the question arises as to which is the more primitive 

 method. If the appearance of individual organs as in Ameiu- 

 rus, each of which is homologous with sensory areas of the audi- 

 tory vesicle, is the primitive method, it is conceivable that the 

 elongation and precocious appearance of the primordia from 

 which these organs appear (and these primordia are always longer 

 than the organs in Ameiurus) might result in a more or less defi- 

 nite line such as Wilson describes in Serranus and Miss Clapp in 

 Batrachus. The growth of the postauditory sensory line would 

 mean nothing more than the appearance of successive organs 

 whose primordia have become continuous. If the differentiation 

 of organs on lines which are extensions of the auditory vesicle 

 is the primitive mode, it is more difficult to understand how the 

 method by which they arise in Ameiurus could be derived from 

 it. It is hard to see how organs that primitively come from a com- 

 mon primordium could later in phylogeny arise singly. The dif- 

 ficulty is increased when we remember that the lateralis Vllth 

 ganglion does not come from the vesicle or any of its derivatives 

 but from the lateral mass (neural crest). Perhaps the greatest 

 obstacle to this view on theoretical grounds arises from the con- 

 fusion which is introduced into current ideas of the primitive 

 relation of the lateral line organs to the sensory areas of the audi- 



THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGY, VOL. 20, NO. 4. 



