Ultrastructure of chromocentres and the question of the nucleolar organiser in a green alga.

Chromocentres of Spirogyra submargaritara, examined under the electron microscope, exhibit three regions; 1) highly condensed, fine fibrillar chromatin, 2) the condensed chromatin with nodules, and 3) the “coarsely granular chromatin”. This is the first account of the ultrastructure of chromocentres. Chromocentres adjoining the nucleolar-organising regions of the N.O. chromosomes are compared with the heterochromatic knob, the type “nucleolar organiser” of Mc-Clintock, on chromosome 9 of Zea Mays, and with the structure regarded as organiser in other monocotyledons. Evidence is put forward that the heterochromatic knobs (chromocentres) of Spirogyra are identical in structure whether found in the N.O. region of the chromosome which lies within the nucleolus. The term “heterochromatin” is discussed; in Spirogyra, this terms as at present defined, would have to apply to the three types of chromatin in the chromocentre and to the chromatin of the nucleolar-organi sing region.