360 F. L. LANDACRE 



lateral line organs in the same region is not positive evidence of 

 genetic relationship between the two structures. The divergence 

 in our accounts begins in the description of the method by which 

 the specific lateral organs arise, more particularly as to whether 

 the sensory ridge on which Wilson finds the lateral line organs 

 differentiating is an extension from the pre- and postauditory 

 placodes. In Ameiurus I find no common sensory ridge from which 

 the organs differentiate, and can trace the gradual disappearance 

 of the pre- and postauditory placodes. Miss Clapp ('99, pp. 239 

 251) states that these sensory ridges originate in the auditory 

 region but nowhere I think traces them to the auditory vesicle 

 or its derivatives, while Miss Beckwith ('07) states definitely in 

 tracing the genesis of the lines that they do not come from the 

 auditory vesicle but arise as local differentiations of the epider- 

 mis on which the lateral line organs later appear (p. 28). 



There is a bare possibility that the relation of the anterior sen- 

 sory ridge to the branchial sense organ as described by Wilson 

 in Serranus is one of contiguity only and not of genetic relation- 

 ship. If this be admitted, and it is the point about which Wilson 

 is least definite, then there are no insuperable difficulties in har- 

 monizing the two views, for the lines seen by Allis ('89) and other 

 workers may be interpreted in two different ways. They might 

 be continuous ridges which break up into individual sense organs, 

 or on the other hand they might be formed by a series of individual 

 sense organ primordia whose extremities become confluent. There 

 is evidence for both views. Wilson is very postitive concerning 

 the mode of formation of the postauditory lateral line organs from 

 a common primordium derived from the postauditory extension 

 of the vesicle in the sea bass, while I have evidence which I do not 

 think can be doubted that even the postauditory lateral line or- 

 gans arise separately and have nothing at all to do with the post- 

 auditory placode. This conclusion was reached by Hoffmann 

 ('94) and by Miss Piatt ('95, '96) in part of the organs in Necturus. 

 The same conclusion is reached by Miss Beckwith in Amia. Allis's 

 work ('89) does not really confirm Wilson, since his earliest 

 stage was about one day old (after hatching) and this is too late to 



