328 THE BIOLOGY OF STENTOR 



many micronuclei depart from their location on or near the macro- 

 nuclear surface and become freed in the endoplasm. All become 

 greatly enlarged and hence less strongly staining. Then occur a 

 series of non-simultaneous mitotic divisions, but increase in the 

 number of micronuclei is overbalanced by the fact that many 

 degenerate even after they have begun the first stages of mitosis. 

 As these divisions proceed, the remaining micronuclei become 

 smaller and fewer in number until there is only one sexual nucleus 

 in each partner. Presumably the penultimate division is the first 

 maturation division and division of the last remaining micronucleus 

 is the second maturation division with reduction in number of 

 chromosomes in the migrating and stationary pronuclei formed. 



The migrating pronucleus then becomes surrounded by a halo of 

 homogeneous, darkly-staining cytoplasm. Nucleus and halo both 

 break through the separation membrane to move across to the 

 partner cell so that there is always a small exchange of cytoplasm. 

 At early stages it appears that this cytoplasm is pulling the nucleus 

 along, later the reverse. Once across, the migratory pronucleus 

 breaks loose from its halo and unites with the partner stationary 

 pronucleus of different appearance. The fertiHzation nucleus then 

 becomes itself surrounded by a thickened halo and undergoes two 

 post-fertilization divisions giving rise to four simpler nuclei. 



Separation of the animals occurs about this time. Two of the 



9. Two nuclei form macronuclear anlagen and two produce 

 mitotic spindles but without showing chromosomes and will 



form 4 micronuclei. 



10. Day after separation i to 10 macronuclear anlagen, 

 depending on combinations of fusion and amitotic increase. 



Four micronuclei. 



11. Two-four days after separation. Anlagen with chromatin 

 net resembling chromosomes, and nucleoli appear. Old macro- 

 nuclear nodes resorb as soon as anlagen attain same size. Micro- 

 nuclear increase by mitosis with clearly defined chromosomes. 



12. Constriction of anlagen into nodes and their attaching 

 together. They are not sorted out between daughter cells. 

 Micronuclei have final size and location but are not yet of 



definitive number. 



13. First fission 10 days after separation of conjugants, with 

 the usual vegetative division of nuclei producing 2 normal 



vegetative stentors (14). 



