The Mechanism of Cell Division in the Cleavage of the Newt's Egg

The early cleavages in eggs of Triturus alpestris have been studied. Ciné-film technique was used to record changes in the shape of the egg and movements of the surface pigment from which measurements of linear and areal changes were made. Local vital staining was also employed. There was no significant net change in the area of pigmented cortex during cleavage. Before cleavage the egg resembles a viscous liquid drop whose shape is maintained by a uniform elastic shell of Young's Modulus 1.5 x 105 dynes/cm.2 and thickness about 2µ. The egg assumes a more nearly spherical shape immediately before cleavage when the flexural rigidity of the surface layers increases. The flexural rigidity of the cortical layers was found to be maximal at the beginning of cleavage and minimal midway between cleavages. This variation is similar to that previously recorded for cleavage in sea-urchin eggs by Mitchison & Swann (1955), using a similar method. At any particular stage with respect to the cleavage cycle no variation was found in the rigidity at different points on the egg surface. Serial sections show cytoplasmic modification below and ahead of the forming furrow. It was concluded that the new unpigmented cortex, by which the daughter blastomeres remain in contact after cleavage, is first formed as a sheet of gel (which in later stages can be seen to be a double layer) which grows downwards by a process involving gelation at its lower edge, through the cytoplasm from the animal toward the vegetal surface. The gel layer is assumed to contract immediately after its formation, and in this way to produce "dipping in" of the new furrow and all the observed surface movements. These ideas have been developed to form a detailed theory of cleavage in the newt, and suggest a common basis for the consideration of cell division in echinoderm eggs, plants and other forms, on the basis that the necessary increase in surface area is achieved by the formation of new cortex rather than by the expansion of the original cell membrane. The authors wish to thank Prof. M. M. Swann and Dr J. M. Mitchison, both of the Zoology department, Edinburgh University, for helpful discussion during the course of this work. The ciné films used in the course of this study were made by Mr E. Lucey of this department. Measurements of the rate of furrowing and much of the work using a local vital staining method was performed by Miss H. Yates of this department. Dr E. Deuchar, now at University College, London, prepared some of the earlier sectioned material, and we thank Dr M. Fischberg, Oxford University, for a suggestion concerning spindle fibres. The work received the financial support of the Agricultural Research Council.