PARKER : LONGITUDINAL FISSION IN METRIDIUM MARGINATUM. 53 



While some may be monstrosities, the occurrence of natural pairs 

 shows that longitudinal fission takes place. 



This process is probably extremely slow. 



While monoglyphic and irregular diglyphic specimens may be pro- 

 duced by this and other non-sexual processes, the production of regular 

 hexamerous diglyphic specimens by non-sexual methods has not been 

 observed. Such specimens number about one fifth of all collected, and 

 may be the products of sexual reproduction. 



Cambridge, January 13, 1899. 



