The frog cell is very big and when divided into two hemispheres, the upper hemisphere is darker and has the animal pole, while the lower hemisphere is lighter and has the vegetal pole.
In amphibians, when the eggs of the female are fertilized by the sperm of the males, then a crescent-shaped grey color area starts developing on the side opposite to the entry of the sperm. This grey crescent is visible on the surface of the egg and occurs due to the stimulation of cytoplasm movement when the sperm enters the egg cell.
In amphibians like Frogs and Salamanders, one can see the development of this gray crescent upon the fertilization and formation of the embryo. The gray crescent structure has the capability of initiating the process of gastrulation in amphibians. Gastrulation is that stage of embryo development where a gastrula is formed by the inward migration of the cells from the blastula stage. All the embryonic development that occurs in the future is dependent on the correct and proper interactions of the cells during gastrulation. If the gray crescent is not formed properly, then the embryos do not develop and often die. A dorsal-ventral axis is formed when there is a rotation of 30º to form the gray crescent upon sperm entry. Hence, the determination of the orientation of a cell and its dorsal or ventral polarity is determined by the gray crescent.
When Spemann separated the single-celled embryo into two halves using microsurgical techniques, the results he obtained were consistent with the results of Driesch. The plane at which the division is done is a crucial point. He observed that when the gray crescent band was bisected equally, then the two halves grew and developed normally. However, when this gray crescent region was not present in either half, the embryo which did not have this morphological feature could not develop properly. The half that had the gray crescent area on the other hand could develop normally. Thus, the importance of the gray crescent band was known for its crucial role in determining the fate of the subsequent cells.