GRADIENT-FIELDS 41 



By making small lesions in the surface of frog eggs, Ancel 

 and Vintemberger (1935) were able to show that the formation 

 of the grey crescent is due to shifting of the egg cortex relative 

 to the deeper layers of the cytoplasm. On the dorsal side, the 

 egg cortex moves toward the animal pole, and also slightly 

 towards the median plane. Part of the superficial pigment gets 

 caught in this movement, and a less pigmented area, the grey 

 crescent, is left behind. 



The ascidian egg is another example of an egg in which 

 bilateral symmetry does not become visible until after fertilisa- 

 tion (p. 58). 



For frog eggs, Ancel and Vintemberger (1948) have also 

 solved the problem of what factors determine the position of 

 the plane of symmetry of the egg. Immediately after spawning, 

 the orientation in space of the amphibian egg is arbitrary. 

 When, however, some time after fertilisation the perivitelline 

 fluid is extruded, the eggs can turn freely in their capsules, 

 and are oriented by gravity. They rotate so that the main axis 

 is vertical, with the animal pole pointing upwards. This rotation 

 proved important for the determination of bilateral symmetry. 

 The meridional plane in which the rotation takes place will 

 later be the plane of symmetry of the egg. The grey crescent 

 will form on the side along which the vegetative pole has 

 descended. The point of entry of the sperm into the egg also 

 plays a part: the grey crescent is formed preferably at the 

 opposite side of the egg. The influence of the sperm, however, 

 is less marked than that of the orienting rotation. Normally, 

 the grey crescent is formed in accordance with the latter. The 

 influence of the sperm will take effect only in eggs whose 

 animal pole accidentally pointed upwards from the beginning, 

 and which therefore did not rotate. Both factors can be 

 eliminated experimentally by fixing unfertilised eggs so that 

 their animal poles point upwards, and activating them by means 

 of an electrical induction shock (p. 20). In this case, the grey 

 crescent may be formed anywhere. This shows that the origin 

 of bilateral symmetry, per se, does depend on the starting of 

 development by fertilisation or by activation, but not on the 

 one-sided penetration of a sperm or on the orienting rotation. 



