Question

Gray crescent in fertilized frog's egg is the area

• A. At the point of the entry of sperm into ovum
• B. At the vegetal pole
• C. At the animal pole
• D. None of the above

Answer 1

The correct option is D None of the above

In some species, the distribution of developmental determinants in the unfertilized egg is maintained in the zygote. In others sperm penetration initiates rearrangement of the cytoplasm. For example, fertilization in the amphibian egg causes a shift of some of the cortical cytoplasm. These cytoplasmic movements can be easily followed because the egg cortex contains dark pigment granules. As a result of this rearrangement, a crescent shaped region of underlying lighter-colored (gray) cytoplasm becomes evident directly opposite the point on the cell where the sperm penetrated the egg. This gray crescent region is thought to contain growth factors and other developmental determinants.The first cleavage bisects the gray crescent, distributing half to each of the first two blastomeres; in this way the position of the gray crescent establishes the future right and left halves of the embryo. As cleavage continues, the gray crescent material becomes partitioned into certain blastomeres. Those that contain parts of the gray crescent eventually develop into the dorsal region of the embryo. The gray crescent is not located on animal pole (relatively active yolk free end of egg) or vegetal pole (relatively inactive yolk laden end of egg).