THE INFLUENCE OF CENTRIFUGAL FORCE ON THE BILATERAL DETERMINATION OF THE SPIRALLY-CLEAVING EGG OF URECHIS

¹ From the William G. Kerckhoff Laboratories of the Biological Sciences of the California Institute of Technology at Pasadena, California

1. The eggs of Urechis caupo were ultracentrifuged for short periods with forces ranging from 6,000 to 75,000 gravities before fertilization and following fertilization at various times before cleavage.

2. In the trochophores 75 per cent of the total recorded embryos had the centripetal oil in the ventral median or oblique position, in slightly more than 20 per cent it was found laterally, and in less than 5 per cent dorsally.

3. The data are essentially the same for unfertilized eggs and all eggs centrifuged at various times following fertilization and before cleavage, and irrespective of the force applied.

4. From these data it is concluded that the bilateral axis of the embryo is induced or influenced by the stratification of material substances in the egg. In centrifuged eggs it is unrelated to either the sperm entrance point or the first cleavage plane.

5. There is no polar orientation in the centrifuge tubes before fertilization or following fertilization until the germinal vesicle breakdown. But following the germinal vesicle breakdown the eggs tend to orient in the centrifuge tubes so that the pole comes to lie most commonly near the centrifugal end.

6. Normal trochophores are produced from eggs centrifuged before fertilization and at certain times following fertilization, but during some miotic and mitotic stages centrifuging suppresses polar body formation and disturbs the cleavage spindles so that few normal embryos are produced.