Electron microscope studies on mitosis in sea-urchin blastomeres

Abstract

The mitotic figure of sea-urchin blastomeres was observed with the electron microscope. (1) The aster seems to arise from a radial orientation of bead-like chains of small cytoplasmic granules (microsomes), finally attaining a tubular appearance probably due to the fusion of neighbouring vesicular microsomes. (2) Astral rays branch and anastomose with each other forming a rather compact reticulum near the center of the aster identifiable with the so-called centrosphere. (3) The centrosome appearing as a single mass of several minute granules is found at the center of the aster. (4) The mode of development and fine structure of the spindle fibers are recognized as similar to those of the astral rays; but the spindle can be assumed to be of nuclear origin. (5) During the disappearing process of the amphiaster, the tubular feature of the fibers fades away turning into the bead-like chains of granules or vesicles, eventually being transformed into dispersed micro-somes. (6) Metaphase chromosomes were observed to consist of an electron dense cortex and a less opaqtie internal matrix without evidence of limiting membranes. The cortex is composed of tightly ranked strands or tubules which may represent chromonemata that tightly surround the chromosome as a spiral. (7) The contact surface of daughter cells is found to be smooth in contrast to the free surface of the fertilized ovum which bears numerous protrusions, leading to an assumption that the cleavage surface may be newly-formed. (8) A preliminary discussion is given on the mechanism of chromosome movement and other functions of achromatic elements, concerning their physico-chemical properties.