1003.] NATURAL SCIENCES OF PHILADELPHIA. 753 



November 17. 



The President, Samuel G. Dixon, M.D,, in the Chair. 



Nineteen persons present. 



The Cause of Inverse Symmetry. — Dr. Edwin G. Conklin remarked 

 that the fact that animals with totally inverse symmetry may occur 

 within the limits of the same species indicates tlVat the cause of this 

 phenomenon must be an ontogenetic rather than a philogenetic one. 

 Furthermore, in all cases in which the development of an inversely 

 symmetrical animal has been studied, the cleavage of the egg is also 

 found to be inverse. He had found that the inverse cleavage of 

 Gasteropods may be traced back to the very first division of the egg, 

 and that this must be preceded by an inverse organization of the 

 unsegmcnted egg. No inverse organization can be detected in the 

 ovarian eggs of sinistral snails, and it is therefore probable that it 

 arises about the time of the maturation or fertilization of the egg. 

 In dextral snails the polar bodies are formed at what was the free pole 

 of the ovarian egg, and if the polar bodies were to be formed at the oppo- 

 site or attached pole in sinistral forms it would entirely and satis- 

 factorily explain their inverse symmetry. While such a reversal of the 

 polarity of the egg in sinistral forms has not been demonstrated, 

 certain observations have been made which render it probable. 



The subject was discussed by ^Messrs. Chapman, Pilsbry and Skinner. 



Morgan Hebard and Henry W. Fowler were elected members. 



Hugo de A'ries, of Amsterdam, Eduard Strasburger, of Bonn, and 

 Nestor Grehant, of Paris, were elected correspondents. 



The following were accepted for publication : 



■is 



