Experimental control of the site of embryonic axis formation in Xenopus laevis eggs centrifuged before first cleavage.

In Xenopus laevis, the dorsal structures normally develop from regions of the egg opposite the side of sperm entry. Gravity is known to affect this topographic relationship in eggs inclined obliquely from their normal vertical orientation in the period before first cleavage. This effect has been explored in detail, making use of low-speed centrifugation (10-50 g) for short durations (4 min). Eggs were immobilized in gelatin and oriented with their animal-vegetal axes 90 degrees to the force vector, with the sperm entry point (SEP) side of the egg either toward or away from the center of the rotor. It has been found that the egg shows three distinct periods of response to centrifugal force in the interval from fertilization to first cleavage: Prior to 0.4 (40% of the first cleavage interval), the egg is very sensitive to centrifugal force and develops dorsal structures from its centrifugal side, regardless of the position of the SEP in the centrifugal field. Thus, the dorsal structures of the embryo are reversed from normal in eggs centrifuged with the SEP away from the center of the rotor. In the period 0.4 to 0.7, the egg is still very sensitive to centrifugal force and develops dorsal structures from its centripetal side, regardless of the position of the SEP in the centrifugal field. Thus, the dorsal structures of the embryo are reversed from normal in eggs centrifuged with the SEP toward the center of the rotor. In the period 0.7-1.0, the egg becomes increasingly resistant to centrifugal force and forms dorsal structures at the normal position opposite the SEP side. This resistance can be overcome in some egg clutches by 50 g centrifugation followed by prolonged 90 degrees off-axis inclination at 1g. Midway in the second cell cycle, there is a brief period of sensitivity to centrifugal force. These These results are discussed in terms of the types of cytoplasmic rearrangements occurring in the egg at different times of the cell cycle, and in terms of the process of cytoplasmic localization of determinants of dorsal axial development.