Nuclear sheets in the egg of a fern, Dryopteris filix-mas.

In the maturing egg of Dryopteris filix-mas the nucleus is shown to produce sheet-like extensions into the cytoplasm, resembling those described in certain somatic cells of animals. In these sheets the distance between the 2 inner membranes of the envelope is regularly of the order of 45-50 nm, and the nuclear material is organized into a monolayer of electron-opaque aggregates 35-40 nm in diameter, occasionally with a central space. Much of the osmiophilic material of the aggregates is removed by pronase and trypsin, leaving granular osmiophilic material, but no fibrils. Ribonuclease and deoxyribonuclease produced little effect. Tritiated thymidine, fed for 6 h before fixation to maturing eggs, could not be shown to be incorporated into the aggregates, despite marked incorporation into plastids and mitochondria. In the absence of compelling evidence that the electron-opaque material is chromatin, it is proposed that the aggregates represent enzyme complexes concerned with membrane growth, believed to be active in the nuclear evaginations. These complexes are not considered to be confined to the sheet-like parts of evaginations, but to be brought into an aggregated form in the sheets as a result of compression, as suggested for the similar organization of chromatin in the nuclear sheets of animal cells.