Session III: The Effeet of Weightlessness on the Dividing Egg of Rana pipiens

The egg of the frog has long been known to respond to disorientation with respect to the force of gravity. The experiments most often referred to as illustrative of this phenomenon are those first performed by Schultze (1894). Others have repeated and elaborated upon these, the most exhaustive studies reported being those of Penners and Schleip (1928). This latter paper also comprises an extensive review and critique of such experimentation as reported up to that time. Under normal conditions the egg, shortly after fertilization and depending upon its initial position, may turn up to 1800 within the jelly capsule ("rotation of orientation"). It comes to rest with its distinctive light-colored, heavy yolk mass (vegetal pole) lowermost. Along this gradient the embryo normally forms. Explanation of the respective roles of fertilization, orientation, and gravity in the determination of bilateral symmetry in normal development has been elucidated and enhanced by the work of Ancel and Vintemberger (1948). If rotation is prevented by compressing the egg between two glass slides and maintaining its vegetal pole uppermost, the formation of twin-headed monsters, as well as other abnormalities, may be induced. This inversion must be applied prior to first cell division to achieve maximum effectiveness of monster formation. Penners and Schleip (1928), however, present evidence to show that in Rana fusca some lesser sensitivity to initiation of inversion persists through the 8-cell stage. It was evident that this gravity-sensitive stage of embryologic development might be adaptable to a study of the biological effects of weightlessness as produced by an orbiting satellite. As the centrifuge is used to study the nature and processes of the cell, the BIOSATELLITE II experiment then was designed with the question asked: Does weightlessness affect the ability of the fertilized frog egg to divide, differentiate, and develop normally?