The Latero-Seiisory Canals and Pielated Bones in Fishes. 477 



of two or more layers. At this stage the sense organs appear as little 

 pits in the bottom of a continous groove, much, it would seem, like 

 the middle head line of pit organs in the adult Esox. In slightly older 

 stages, according to Coggi, the sense organs themselves also become 

 covered by the overgrowing of the superficial layer or layers of the 

 ectoderm, and from that moment the development of the canals takes 

 place much as Balfour has described it, excepting that the canals are 

 said to first form on the head instead of in the tail region. 



In Amia the latero-sensory canals are developed in a manner quite 

 different from that just above set forth for plagiostomes. In this fish^the 

 canals are formed by the involution of a strip of epiblast that would seem 

 to compare with the region that takes part in the formation of the 

 canals in plagiostomes much as a valley corresponds to a river bed that 

 traverses it. The canals are, in this fish, first enclosed on the anterior 

 part of the head, as Coggi says they are in Torpedo, instead of near the 

 tail, as Balfour and Solger both affirm for plagiostomes in general. 

 The canals are, in Amia, uninterruptedly in communication with the 

 exterior by the surface openings of their primary tubes; and the sense 

 organs are large and relatively few in number, while in the plagio- 

 stomes they are small and numerous, whole groups of organs in these 

 latter fishes apparently corresponding to single organs in Amia. The 

 organs of Amia, although apparently developed from and in the deeper 

 layers only of the epiblast, always break through to the outer surface 

 before they become enclosed in the canals, and they lie between the 

 points of exit of two adjoining primary tubes, instead of opposite 

 those tubes. 



The early stages of the development of the latero-sensory system, 

 that is the development first of the sensory lines and then of the in- 

 dividual sense organs, I did not trace in Amia, but it would seem as 

 if it must be similar to that described by Wilson [79] in Serranus. 

 In this fish an involuted groove of sensory tissue is said to first de- 

 velop in the deeper layers only of the epiblast, this stage much re- 

 sembling the condition described by Balfour in plagiostomes. But this 

 groove, in Serranus, never becomes a canal. It flattens out as it were, 

 in certain places, and sense organs there arise. Between these organs 



