Cleavage furrow formation in a telolecithal egg (Loligo pealii). II. Direct evidence for a contraction of the cleavage furrow base

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Abstract
Cleavage of the highly telolecithal egg of a squid is meroblastic and stops when the base of each furrow cuts through the cytoplasm of the blastodisc and contacts the yolk. The base of each furrow is composed of a band of 50 Å filaments which apparently are linked to the surface membrane. By cutting the base of the furrow or by flattening the blastodisc of the egg, it has been possible to demonstrate that this band contracts and remains in a state of tension. Analysis of the shape change of the cells in normal cleavage also supports this contraction‐tension hypothesis. Furrows which cross another furrow are linked to it at specific sites (anchor points). As the furrows extend, new linkages between the filamentous band and the surface membrane are formed. The amount of tension produced by contraction of a furrow is proportional to its length. It appears that the entire blastoderm is in a state of dynamic tension caused by balanced contraction of the filamentous band. These results and conclusions are in agreement with postulated mechanisms of cleavage furrow formation in many other types of animal cells.