iJUSTELUS LiEVlS. 95 



depressiou leading to the outer surface of the head. At 

 and immediately before this stage this organ would strongly 

 resemble in general appearance the ampullary pits in the 

 temporal region of my 55 mm. Mustelus. It would also 

 present somewhat the appearance of the pit-like depressions 

 that I have assumed to represent the future pores of the 

 canal lines in 3(3 mm. embryos of Mustelus, but my material 

 was not suited to the determination of the homologies here 

 involved. All that I could make out was that the canal organs 

 seem never, in Mustelus, to become exposed on the outer 

 surface of the head, as they do in Amia. On the contrary, 

 they seem to always remain in the deeper layer of the 

 ectoderm, wliere they arise, and then to split off from that 

 layer, enclosed in, and as part of, a long and nearly solid 

 cord, which later becomes a canal. The ampullary organs, 

 in somewhat marked distinction to the canal organs, may 

 become exposed at the bottom of a little pit apparently formed 

 by the bursting, so to speak, of the little vacuole that forms 

 above the central portion of the organ. The general form and 

 appearance of the several organs in my embryos thus give no 

 definite indication as to whether the ampullar are developed 

 from pit-organs or not. They, however, seem to indicate a 

 difference between ampullary and lateral canal organs. 



No organs in any way resembling the terminal buds of 

 ganoids and teleosts were anywhere observed on the outer 

 surface of any of my specimens, but in the large number of 

 sections examined, and especially as I Avas not particularly 

 looking for these organs at the time, it is certainly possible 

 that there may have been some, and that they escaped my 

 notice. There were, however, in the 12-2 cm. embryo certain 

 other sensory organs found both on the outer surface of the 

 head and on the body. They closely resemble the organs 

 that in my 55 mm. embryo represent certain of the super- 

 ficial ophthalmic group of ampulIfB, but differ from those 

 organs in that their central lumen in every case leads directly 

 to the outer surface by a pit-like depression. The distribution 

 of these organs will be given in describing the ampuUse. 



