170 J. J. LISTER, 



spheric. On the hypothesis that the two forms represent dif- 

 ferent sexes, we should expect to find both megalospheric and 

 microspheric young produced by the microspheric parent. 



Adopting this view, the individuals of the microspheric 

 generation of Polystomella give rise asexually, by a process 

 of multiple fission, to young megalospheric forms, and these 

 in their turn produce, also by multiple fission, the flagellate 

 zoospores. 



In Orhitolites the microspheric form gives rise to megalo- 

 spheric young, but in this genus, as in Peneroplis and Rqtalia, 

 the megalospheric form may repeat itself for one or more gene- 

 rations, though ultimately it may be assumed by analogy with 

 Polystomella, that a megalospheric form is produced which 

 gives rise to zoospores. 



How is the gap in the life-history between the liberation of 

 the zoospores and the formation of the microsphere filled in ? 



ScHAUDiNN has recently described {Sitz-Ber. Gesellsch. 

 Naturforsch. Freunde, Berlin, Jahrg. 1894. Abstract in 'Zool. 

 Centralblatt,' I., p. 519) the production of zoospores of approxi- 

 mately uniform size, in Hyalopus (Gromia dujardinii, M. ScH.) 

 and their subsequent conjugation. 



It appears not improbable that a similar process may occur 

 in Polystomella and the other dimorphic species, the initial 

 chamber of the microspheric form being the result of the con- 

 jugation of zoospores of the megalospheric form. The sizes of 

 the zoospores and microsphere in Polystomella (about 4 /t and 

 6-13 fi, respectively) accord fairly well with this view ; and the 

 comparative scarcity of the microspheric form may be under- 

 stood on the supposition that the union of two separate organ- 

 isms is required for its production. 



If this is the case, the two forms of the Foraminifera must 

 be regarded as sexual and asexual generations, of which the 

 sexual generation retains the power in some genera {Orhitolites 

 and others) of reproducing asexually. 



It must, however, be borne in mind that while there seems 

 good reason to regard the two forms of Foraminifera as belong- 

 ing to different generations, the view above suggested of the 



