94 EDWAED PHELPS ALLtS, J UN. 



side of a lateral canal opposed to tliat on which the ampulh^ 

 themselves lie, the ampuUary tubes passing internal and not 

 external to the lateral canal. As the lateral canals in Mnstelus 

 are certainly already present as well-developed cords before 

 the arapullary tubes are developed, it is evident that the 

 latter tubes, in the cases above referred to, would have had to 

 cut through the canal to attain their adult position, if the pore 

 of the tube travelled from its place of origin in anything 

 resembling the manner that the lateral pores of Amia do (2). 

 An extreme case is shown in Garman's (21) figure of Raia, 

 where long ampullary tubes open along the edge of the body 

 lateral to the lateral canals, the tubes passing internal to the 

 canals. The ampullts related to these tubes and pores must 

 certainly have had their place of origin in the immediate 

 neighbourhood of these pores of the adult, and from there 

 they must have travelled to their adult position by pushing, 

 or being pulled, through the tissues internal to the canals. 



The topographical position of a group of ampullas in the 

 adult selachian is thus not necessarily any indication what- 

 ever of the point of origin of the several ampullaj that form 

 the group, the ampullary organs differing radically in this 

 from the organs of the lateral canals. They also apparently 

 diifer from the latter organs in that they have a later and 

 relatively much more rapid development. This is shown in 

 the rapid development of the ampullary tubes just above 

 referred to, between the ages represented by the 55 mm. and 

 12*2 cm. embryos. In my 36 mm. embryo I could not even 

 find any positive indication of this line. 



In Amia (2), and probably in all teleosts also (67), the 

 sense organs of the lateral system lie at first below the 

 outer surface of the ecioderm, along a cord of cells that is 

 differentiated in the deeper layer or layers of the ectoderm. 

 Immediately superficial to the central point of each organ 

 there is a large and specialised cell, which later becomes a 

 vacuole. As the -sense organ pushes through the overlying 

 cells to its final exposed relation to the outer surface, this 

 vacuole must, at a certain stage, become a small pit-like 



