On the Mechanism of Astral Cleavage

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Abstract
Classical theories of cell-cleavage mechanics[long dash]traction-fiber (Heiden-hain ''97), surface tension (Rhumbler et al.), astral gelation (Gray ''24) and equatorial surface contraction (Chalkley et al.)[long dash]are analyzed and found inadequate to explain observed facts. Studies of the behavior of the surface layers of echino-derm and other marine eggs by Dan and co-workers have been carried out by 2 principal methods. Following the position of recognizable particles affixed to the surface of cleaving eggs, Dan finds a consistent surface shrinkage at the site of the future cleavage furrow accompanying the first departure from the spherical state. This is succeeded by a tremendous expansion as the furrow deepens. Similar observation of (Ilyanassa) eggs with "trefoil" cleavage shows that the underlying spindle-aster system is a prerequisite for such shrinkage. As the result of this and another series of expts. in which perforations were made in dividing eggs and the shape of these holes followed through cleavage, Dan. concludes that Gray''s concept of the aster as a simple sphere of gelated protoplasm must be modified to that of a spherical structure composed of spines radiating from a central core, enclosing fluid endoplasm in their interstices. Using the data of these expts. and those of other workers, especially the observation that fully developed astral rays cross in the midplane of the cell, a theory of the mechanism of astral cleavage is formulated, based on the assumptions that (1) the asters are radiate spheres; (2) astral rays are anchored in the egg cortex; (3) the spindle elongates autonomously, causing the rays crossing each other in the midplane to pull the cortex together until (4) they are finally pulled loose from the cortex as the asters are pushed apart. Since the volume of the cell remains constant, the fluid endoplasm becomes insufficient to fill the interstices of the now discrete astral spheres as well as the space between them, and (5) a suction force is developed which pulls in the unsupported equatorial region and forms the cleavage furrow. Data accumulated by various workers are adduced to test the validity of this proposition, and a single-plane model is constructed to illustrate it mechanically. Adaptation of the theory and model to the atypical case of cells with excentric spindles shows that the extended spindle bends in response to the unequal pull of long and short rays. Examination of eggs naturally exhibiting such excentricity shows that the spindle is actually bending, and in proportion to the degree of excentricity. Dan concludes that the cause of spindle-bending lies in the one-sided pressure of the advancing animal polar furrow against the sheath-rays which transmit it to the spindle. Exptl. production of such an unbalanced condition confirms this conclusion. Study of the completely one-sided cleavage of medusan eggs shows astral rotation and spindle bending present in an extreme degree. A marked fluidity of the vegetal region of these eggs causes an even greater unbalance than could be attributed to nuclear excentricity alone. It is concluded that two graduated series are discernible: one depending upon excentricity of the nuclei and the other upon difference in rigidity between the animal and vegetal cortical regions.
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