Question

Gray crescent structure is found in

• A. Eye of cockroach
• B. Eye of frog
• C. Brain of rabbit
• D. Fertilized egg of frog

Answer 1

The correct option is D Fertilized egg of frog

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-coloured (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.