FINE-STRUCTURE OF CONJUGATION OF THE CILIATE BLEPHARISMA-JAPONICUM - I - CHANGES OF THE OLD MACRONUCLEUS

The macronucleus of vegetative cells of Blepharisma japonicum contains numerous discrete chromatin bodies and nucleoli of the compact type. The nucleoli contain very dense inclusions likely to be the dense fibr illar component of the nucleoli themselves. The macronuclear envelope displays tightly packed pore complexes. Until the 10th hour of conjuga tion, the old macronucleus shows the same fine structure as in vegetat ive cells. At 12 hours (approximately during metaphase I), the fibrill ar and granular parts of the nucleoli segregate and the latter gradual ly disappear. At 14 hours, all nucleoli are reduced to their fibrillar parts that have the aspect of small dense bodies scattered in the nuc leus. Simultaneously, the chromatin bodies begin to unite into a commo n network. This process continues at 16 hours (stage of pronuclei). Th e chromatin network is connected to the nuclear envelope via submembra ne bodies of condensed chromatin tightly applied to the inner nuclear membrane. The macronuclear envelope has no coating of dense material i n these areas. Thereafter (24 and 26 hours), areas of nuclear matrix f ree of the chromatin network appear in the macronucleus. At 28 and 34 hours, the chromatin network condenses and the residual nucleoli are c aught in its meshes, being incorporated into the chromatin. The points of contact of the network with the nuclear envelope become less numer ous.