Question

Grey crescent is
A. Melanin rich area of egg
B. Melanin rich area formed at the region opposite to entry of the sperm
C. Yolk rich area of egg
D. Laying of sperms and eggs in Frog

Answer 1

Hint: Frogs are greenish yellow coloured amphibians and the zygote of frog contains grey crescent. Grey crescent is the area between the two poles, i.e. between vegetal and the animal pole.

Complete Answer:
Let's discuss the option and find out the correct answer.
Option A: Grey crescent, it is a structure formed on the opposite side of sperm entry in a frog egg. It has a bunch of growth factors which basically regulates the part of the cell becomes which part of the body. After fertilization on the side opposite of sperm penetration a cortical region of the newly fertilized egg of frogs and some salamanders are formed. Therefore, this is the incorrect option.

Option B: Melanin rich area formed at the region opposite to the entry of the sperm: In the zygote of the frog, there are two poles present. The vegetal pole and the animal pole. These are separated by an area which is rich in melanin and is known as the grey crescent. Therefore, this is the correct option.
- The egg enters opposite to the region of sperm entry. The broadest part of the grey crescent forms the dorsal end. The grey crescent is 'Melanin rich area formed at the region opposite to the entry of sperm'.

Option C: Among animals which produce egg, yolk is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. In eggs containing a moderate or large quantity of yolk, the distribution of yolk is not uniform. It is concentrated more towards the vegetal pole. The grey crescent is not located on a vegetal pole, its relatively inactive laden end of egg. Therefore, this is the incorrect option.

Option D: Eggs of egg laying mammals have a huge amount of yolk, much greater than the cytoplasm present in the cell (Macrolecithal). Based on the distribution of yolk, they are where yolk is unevenly distributed. Hence option D is incorrect. Therefore, this is the incorrect option.

Thus, the correct option is B) Melanin-rich area formed at the region opposite to the entry of the sperm.

Note: The importance of grey crescent is,
- It marks the future dorsal side of the embryo.
- When the first cleavage happens, the grey crescent is divided into two equal halves and this represents the future median plane of the embryo.