608 MAEOUS M. HAETOG. 



have hence two meganuclei; and it is only at the second 

 bipartition that the nuclear apparatus is reduced to one mega- 

 and one micro-nucleus as characteristic of the species. The 

 primitive meganucleus of the gametes undergoes fragmen- 

 tation into as many as sixty parts, which persist in the ex- 

 conjugates and their offspring for some time; they usually 

 undergo absorption, or are expelled as faeces ; but if the 

 culture is starved, some may fuse with the new mega- 

 nucleus. 



Paramoecium aurelia, with which P. caudatum is 

 usually confounded, has two micronuclei, so that at stage C 

 eight nuclei of the formula ^2 are present ; only one of these 

 undergoes the next bipartition to form the pronuclei, all the 

 others being '' corpuscules de rebut." Stages F and G are as in 

 Colpidium ; but at the first bipartition the micronuclei divide, 

 one daughter nucleus of each going to either of the offspring. 

 The following equations represent Stage H and the first 

 fissions of Paramoecium aurelia: 



4Z2 = 2in-f2M (Stage H). 



2m + 2M = 4mM-2M. __^_ 



= {2mi-t-MJ + {2mi + M} (1st fission). 



Still more complicated is the process in Vorticelliiies where 

 the gametes are of unequal size, the smaller ones being formed 

 by vertical fissions of an individual into four or eight, and 

 swimming freely. One of these microgametes conjugates 

 with a megagamete and fuses completely with it, so as to 

 form a single zygote — a process first fully described by Stein. 

 Either gamete here only forms a single functional pronucleus, 

 their sister pronuclei aborting. As the two pronuclei advance 

 to meet they might, perhaps, be regarded both as equivalent 

 to the male pronuclei of other species ; but it is impossible 

 to regard the pronuclei as really differentiated into distinct 

 sexes. Stage G is double, so that eight nuclei are formed. 

 Of these, seven evolve into meganuclei; one into the micro- 

 nucleus of the zygote. Successive fissions of the zygote take 



