Force Exerted by the Cleavage Furrow of Sea Urchin Eggs

A drop of ferrofluid injected into the center of a dividing sea urchin egg is deformed into the shape of an hourglass when the cleavage furrow advances. The force applied to the drop is determined from the deformation of the drop and the interfacial tension between the ferrofluid and the protoplasm. The interfacial tension is determined from the deformation of a spherical drop in the protoplasm when a magnetic field is applied, and the force applied to the drop, which is estimated from the deformation by magnetic field of a similar drop in 2 per cent aqueous solution of Triton X-100 and the interfacial tension between the ferrofluid and this solution. The force applied to the drop in the dividing egg increases during an early stage of cleavage and decreases during a later stage. The force attained a maximum of 9 × 10−3 dyne in an egg of Temnopleurus toreumaticus which pinched the drop into two when it divided. Smaller maximum forces, 3.9 × 10−3 dyne in the eggs of Temno-pleurus toreumaticus and 2.0 × 10−3 dyne in the eggs of Clypeaster japonicus (mean values), were obtained when the furrowing was arrested by the drop. The magnitude of the maximum tension developed in the contractile element located in the furrow cortex is discussed.