so 



PROTOZOA 



undergoes three successive divisions, or, strictly speaking, two 

 fissions producing four nuclei, of which one only undergoes the 

 third. The other three nuclei of the second fission degenerate 

 like the meganucleus.^ Of the two micronuclei of this last 

 division one remains where it is as a " stationary " pairing 

 nucleus, while its sister passes as a " migratory " pairing-nucleus 

 into the other mate, and fuses with its stationary pairing-nucleus. 



Fig. 54. — Four individuals of Golems hirius (Gymnostomaceae) swarming about and 

 ingesting a Vorticella (?) (From Verworn.) 



Thus in either mate is formed a " zygote-nucleus," or " fusion- 

 nucleus." All these processes are simultaneous in the two 

 mates ; and the migratory nuclei cross one another on the 

 bridge of junction of the two mates (Fig. 52, C). Each mate now 

 has its original cytoplasm (subject to the qualification above), 



^ When there are at the outset two or more micronuclei all undergo the first 

 two fissions, but only one undergoes the tliinl. 



