Germinal Disc Diameter: Fertile vs Infertile Egg

Correct Answer: 4.5 mm (fertile) & 3.5 mm (infertile)

The germinal disc of a fertile egg is larger (4.5 mm diameter) compared to an infertile egg (3.5 mm diameter). This size difference can be used to identify fertilisation status of an egg.

Blastoderm vs. Blastodisc

• Blastoderm (fertile egg): 4.5 mm diameter; appears as a white disc with a clearly defined outer ring — because fertilization has occurred and cells have divided to form a multi-layered blastoderm.
• Blastodisc (infertile egg): 3.5 mm diameter; appears as a small white spot without a clear ring — no fertilization has occurred.

Memory Tip

Blastoderm (fertilised) = Disk is Depth (bigger, 4.5 mm). Blastodisc (infertile) = smaller disc (3.5 mm).

Why Other Options Are Wrong

• 3.5 mm & 4.5 mm → reversal of the correct order (fertile is larger, not smaller).
• 6.5 mm & 5.5 mm → overestimate; actual diameters are 4.5 mm and 3.5 mm.
• 4.5 mm & 5.5 mm → the infertile egg cannot be larger than the fertile one.