THE CRANIAL GANGLIA IN AMEIURUS 357 



fourth lateral line organ and gives rise to no organs. If any do 

 arise from it it would be the fifth of the body lateral line. 



As to the mode of origin of the organs, there seems to be little 

 variation. Each one appears as a slight thickening of the epi- 

 dermis scarcely perceptible at first except by its location at the 

 seat of the future organ. This thickening becomes more pro- 

 nounced by the elongation of the deeper cells and by their assum- 

 ing a radial arrangement with frequently a small circular cavity 

 just beneath the outer flattened layer of epidermis and at the 

 apex of the radially arranged cells. This process of the elonga- 

 tion of the radially arranged cells continues until the organ assumes 

 its permanent form. The variation mentioned above consists 

 in the different rates at which this process is carried on and also 

 the different stages of development at which the organs sink into 

 canals. The facts brought out above differ materially from the 

 usual description of the relation of the dorso-lateral placode to the 

 lateral line organs and lateral line nerve but are in close agreement 

 with the work of Miss Piatt on Necturus ('95). The fact that 

 anterior to the ear the preauditory dorso-lateral placode disap- 

 pears more than 26 hours before lateral line organs can be detected 

 and that posterior to the ear at least four lateral line organs 

 appear anterior to the placode, while it can still be recognized 

 as such, leaves no doubt in the writer's mind that there is no 

 evidence of a genetic relation between the dorso-lateral placodes 

 and lateral line organs in Ameiurus. The lateral line organs are 

 definite differentiations of the epidermis, just as are the taste 

 buds, and the nerves which supply them grow from specific gang- 

 lia just as in the case of the gustatory nerves. There seems to 

 be no doubt that the auditory vesicle and pre- and postauditory 

 placodes are homologous, or rather the same structure, but the 

 relationship of the lateral line organs and vesicle is on a somewhat 

 different footing. 



If the auditory vesicle phylogenetically is to be looked upon as 

 containing sensory areas homologous to lateral line organs, in the 

 ontogeny we have the troublesome fact that lateral line organs 

 arise in the epidermis in the same area as that from which the 

 vesicle arose; further than this the preauditory lateral line organs 



