244 



Mr. E. J. Bles. On the Openings in the 



development of almost all Vertebrates. This is in agreement with 

 the result arrived at by van Wijhe in a totally different way, which 

 states that the abdominal pores were excretory ducts and the body- 

 cavity the most primitive excretory organ, functioning as such 

 before the pronephros arose,* It would follow that the abdominal 

 pore is phylogenetically older than pronephric and mesonephric seg- 

 mental tubes and ducts, and Balfour's homology of the abdominal 

 pores with a posterior pair of segmental tubes would fall to the 

 ground. Without committing myself to van Wijhe's way of regard- 

 ing the abdominal pores and pronephros, I must say that there is 

 veiy little to be alleged in favour of Balfour's homology. The argu- 

 ments with which Balfourf supports his view are rather scanty, 

 amounting to the statement that the pores, for reasons given, are not 

 Miillerian ducts, and that the blind pockets (cloacal pouches) of 

 Selachians are very like primitive involutions from the exterior to 

 form the external openings of a pair of segmental organs. It is now 

 known that the peritoneal end of segmental tubules is formed, 

 especially in Selachians, in a perfectly definite manner from a defi- 

 nite part of the myotome, the dorsal portion of that part which does 

 not form the myomere or muscle segment. Until it is shown 

 that the abdominal pores arise from a corresponding portion of a 

 myotome they cannot be homologised with segmental organs. Their 

 late appearance in many cases, their ventral position, the fact that 

 they are formed (in Scyllium for instance) at the extreme tip of the 

 peritoneal cavity as a prolongation of that cavity ventrally into the 

 cloacal papilla, that they open at or near the tip of the papilla and 

 not into the bottom of the cloacal pouch, all these facts make it seem 

 unlikely that Balfour's suggested homology will eventually be proved. 



The problem naturally arises, which is the primitive condition in 

 Elasmobranchs, that where nephrostomes alone are present, or that 

 where abdominal pores alone are present in the adult ? This point 

 cannot, I think, be decided without more evidence. 



At first sight it might seem that the persistence of nephrostomial 

 tubes in the adult Selachian was primitive, as these organs are 

 formed at an early stage, in a primitive manner, and are hence 

 phylogenetically ancient structures. Cestracion, moreover, which 

 has this feature, is one of the most primitive of living Selachians. 

 But against all this it may be urged that just as the nephrostomes in 

 adult Amphibia are in all probability a neotenic character, in a 

 group which shows so strong a proclivity to neotenia, so may the 

 retention of open nephrostomes in adult Selachians of the present 

 day be a neotenic phenomenon. This is the more probable, since the 

 nephrostomes in animals which lose them generally disappear at a 

 * ' Arch. L Mikr. Anat.,' vol. 33, p. 507. 

 f Memorial Edition, vol. 1, p. 153. 



