662 E. PH ALUS, 



sensory tissue of which this organ or patch is formed is less tall than 

 the adjacent non- sensory tissue, the sensory patch lies, like the pit- 

 organs of Amia and teleosts, at the bottom of a little pit. The sub- 

 dividing ridges that grow up through the patch thus first cut the 

 organ and then the pit into two or several parts, each ridge thus 

 finally becoming a separating partition, at first relatively thin but 

 often gradually thickening until a relatively considerable portion of 

 the outer surface of the skin lies between the two or more parts of 

 the bisected or multisected organ. A group of organs each resembling 

 the single nerve-sacks of Merkel's descriptions (15) of Acipenser 

 would thus arise, that author's fig. 5, tab. 5, of a transverse section 

 through such an organ being, as far as my material shows, an ex- 

 cellent illustration of a transverse section through one of the organs 

 of Folyodon that is in process of Subdivision. In Acipenser^ however, 

 Merkel says (p. 37) that the separating partitions do not rise to the 

 level of the outer surface of the skin, the primary depressions or pits 

 thus never undergoing complete subdivision, and several nerve-sacks 

 accordingly opening on the outer surface by a common opening. 



The nerve-sacks of Acipenser are considered by Merkel as the 

 homologues of the ampullae of selachians. These latter organs each 

 arise, according to Minckert (16), from a single epidermal sensory 

 thickening. This single patch of sensory tissue, at a certain period, 

 sinks rapidly beneath the surface, and then must first of all acquire an 

 annular disposition around the future centrum of the organ, for both 

 Merkel's (p. 44) and Peabody's (17) descriptions indicate that the 

 flat top of the centrum is non-sensory. The centrum thus probably 

 pushes up through an overlying layer of sensory tissue exactly as the 

 bisecting or multisecting ridges of the primitive pores of Polyodon do. 

 Still later, the resulting ring of sensory tissue becomes cut up into 

 several portions by the growth of partitions radially arranged around 

 the centrum, each partition undoubtedly pushing upward through over- 

 lying sensory tissue, thus completing the analogy with what I conclude 

 to be the manner of development in Folyodon. 



Polyodon, Acipenser, and selachians thus present three successive 

 stages in the development of the ampullae; and Merkel considers all 

 nerve-hillocks of these or other fishes as a fourth and still earlier 

 stage. This latter conclusion I would accept only in so far as it 

 applies to those nerve-hillocks of certain fishes that are said (11) to 

 be innervated by the communis fibres of one or the other of the 

 several cranial nerves; the ampullae of selachians being, in my opin- 



