476 Edward Phelps Allis jr., 



canal is formed by a dehiscence or deliquescence that takes place in 

 the central portion of a previously solid rod formed by the thickening of 

 the under layer (der unteren Lage) of the epidermis. According to 

 both authors the space that represents the canal first appears toward 

 the hind end of the trunk, and from there extends forward. The pri- 

 mary tubes that lead, in the adult, from the canal to the external sur- 

 face are said to develop as solid and even branching rods, which later 

 open on the outer surface fr-om within outwards, by a process of de- 

 liquescence and perforation combined. According to this theory of 

 development it is evident that the epithelium that is said to line the 

 selachian canal must be a differentiation of the cells of the mucous 

 layer of the epiblast, the original epithelial layer of the embryo being 

 excluded fiom the canal by the very manner of its development. 

 According to Solger the sense oigans in the canals do not become 

 evident until after the formation of the primary tubes, and they first 

 appear as a differentiation of the wall of the canal opposite the point 

 of origin of those tubes. Ewart [36], also, shows the organs approxi- 

 mately opposite the inner openings of the tubules, and he says that 

 in Laemargus and Raia (p. 69) the nerves that innervate the organs, 

 "as a rule", there enter the canals. I, however, found, in embryos of 

 Mustelus, the nerves that innervate the organs entering the canals 

 between the points of origin of the tubules, which would seem to in- 

 dicate that the organs, in this fish, also had that position, instead of 

 lying opposite the tubules, 



Coggi says [23, p. 326] that Balfour, and hence Solger also, are 

 wrong in saying that it is the mucous layer only of the ectoderm that 

 takes part in the formation of the lateral canals of selachians. Accor- 

 ding to him [22 and 24], when the four main latero-sensory canals are 

 first evident in Torpedo, the entire ectoderm is still composed of a 

 single layer of cells. The latero-sensory lines, at this stage, are re- 

 presented simply by lines or cords of tall cylindrical cells, which ex- 

 tend through the entire ectoderm. Portions of these cords then become 

 differentiated as sensory organs, and the portions between these organs 

 become covered by the pushing over them of the superficial layer of 

 the surrounding ectoderm, which ectoderm has now become composed 



