Question

Grey crescent develops in
(1) The eye of frog
(2) The fertilized egg of frog
(3) The brain of a rabbit
(4) The eye of a cockroach.

Answer 1

Hint: Grey crescent is a grey coloured crescent like area that develops on the surface of an amphibian egg. The area is present opposite to the point of sperm entry. This feature develops as a result of cytoplasmic movements stimulated by entry of sperm into the egg. Grey crescent area marks the dorsal side of the embryo. The formation of grey crescent is very important as it makes the final symmetry of the egg and the future embryo.

Complete explanation:
Eye of frog: Frogs have three eyelids, of which two are transparent and one semi-transparent called the nictitating membrane. Frogs have the ability to regenerate structures of the eye after damage Frogs swallow food with the help of their eyes. Grey crescent does not develop in frog eyes.
Option (1) is wrong.
Fertilized egg of frog: In developing embryos of frog egg the vegetal pole and the animal pole are separated by an area called the grey crescent. It is present opposite to the region of sperm entry. The grey crescent is very important as it helps in establishing polarity of the embryo.
Option (2) is correct.
Rabbit brain: The brain of a rabbit is formed on a plane similar to that of other vertebrates but is more complicated. Rabbit brain is enclosed in the cranial cavity of the skull. It is surrounded by meninges. The meninges are made up of pia mater, dura mater and arachnoid layer. Grey crescent is not found in a rabbit's brain.
Option (3) is wrong.
Eye of cockroach: The eye of cockroach is composed of multiple photoreceptor units called ommatidia. The ommatidium helps in constructing the image of the object seen. Due to large ommatidia, the cockroach's eyes have a large angle of view and are able to detect small activities by a single glance. Grey crescent is not found in cockroach eyes.
Option (4) is wrong.

Hence, option (2) Fertilized egg of frog is the correct answer

Note:
It is known that grey crescent fixes up the final symmetry of the egg and the embryo. During the gastrula phase, the grey crescent materials are located on the dorsal lip of the blastopore. The main function of the grey crescent is that it acts as the organizer in the embryo. Hence, it is very crucial for embryo development. During the later gastrula phase the grey crescent materials get incorporated with the chorda mesoderm.