GRAVITY INFLUENCES THE POSITION OF THE DORSOVENTRAL AXIS IN MEDAKA FISH EMBRYOS (ORYZIAS-LATIPES)

To determine whether gravity influences the plane of bilateral symmetr y in medaka embryos, zygotes were placed with their animal-vegetal axi s orientated vertically and with their vegetal pole elevated. Then, at regular intervals during the first cell cycle, the zygotes were tilte d 90 degrees for. about 10 min and subsequently returned to their orig inal orientation. In embryos tilted during the first half of the first cell cycle, the embryonic shield formed on the side that had been low ermost when the zygote was tilted. In embryos that were tilted twice, first in one direction and then in the opposite direction, the embryon ic shield formed on the side that was lowermost the first time. When z ygotes were centrifuged at 5 g, the embryonic shield formed on the out wardly radial (centrifugal) side of the embryo. The orientation of the array of parallel microtubules in the vegetal pole region was also in fluenced by tilting or centrifuging zygotes. No correlation was found between the positions of the polar body and the micropyle and the plan e of bilateral symmetry. It was concluded that gravity influences both the plane of bilateral symmetry and the orientation of microtubules i n the vegetal pole region of medaka embryos.