Conjugation in Paramecium Bursaria between Animals with Very Different Chromosome Numbers and between Animals with and without Micronuclei

Virginia were used. All these animals have similar micronuclei. During conjugation the pronuclei in the two conjugants are alike. Under such circumstances evidence of exchange of pronuclei was less convincing although there were many pairs in which an exchange evidently occurred. The migratory pronucleus was usually slender, the stationary pronucleus relatively short and broad. In many cases, the migratory pronuclei were observed crossing from one conjugant to the other. Exceptional cases found in the conjugation between animals having similar micronuclei further strengthen the evidence of exchange of pronuclei in P. bursaria. There is usually a simultaneous movement toward each other of the two migratory pronuclei in the two conjugants, but there are a few exceptional cases in which the movement is not simultaneous. One pronucleus completes its migration by the time the other begins. Such a behavior gives rise to pairs in which one conjugant appears to have three pronuclei whereas the other has one, clearly indicating that an exchange of pronuclei takes place during conjugation in P. bursaria.