Contractility and the fibre systems of Stentor coeruleus.

Observations of the microtubular 'km' system and filamentous ' M ' systems in living specimens of Stentor coerulens show that on contraction of the body both systems shorten, thicken and remain straight, but that on initial relaxation they behave differently: the km fibres begin to lengthen later than the M fibres, the latter being thrown into sinuous folds. Electron microscopy of specimens cooled before fixation appears to confirm this difference in behaviour in relaxing specimens. The M fibres of the stalk region are discrete and prominent bundles, but in the adoral half of the body they are extensively linked together by side branches. There are diffuse filamentous attachments between the M fibres and the kinetosomes. The vertical microtubular stacks which make up the km fibres are each linked with a pair of kinetosomes. Each stack contains about 21 microtubules grouped in a 2 + 19 pattern. Crossbridges are present between the microtubules of adjacent stacks. The number of stacks in each km fibre cross-section is smaller in extended specimens than in contracted ones, indicating that the stacks slide upon one another as the body changes its length.