In the textbook frog, fertilisation occurs externally in water. Sperm and ova meet outside the female’s body. This is distinct from amplexus, the holding behaviour that brings the adults into a suitable position for release. The eggs develop into embryos and hatch into tadpoles, which later undergo metamorphosis to become young frogs.

The tadpole is a larval stage with a body plan suited to aquatic life. The transition to the adult includes major reorganisation, such as limb development and loss of the larval tail, together with changes in respiratory and feeding organisation. Metamorphosis is therefore not merely an increase in size. Adult descriptions such as tailless body and the principal adult respiratory surfaces should not be applied unchanged to the larva.

External fertilisation and indirect development describe different features: the first gives the location of gamete fusion, while the second recognises a larval stage. One term does not automatically define the other for every animal species. This frog happens to show both.

For supplied survival models, apply fractions successively to the appropriate stage. If E eggs are laid, fraction f is fertilised and fraction s of those reaches a later stage, survivors equal E×f×s. Equal final counts can arise from different initial clutches and different success rates.

Worked example: 2000 eggs with 75% fertilisation give 1500 fertilised eggs. If 40% of those reach the stated stage, 600 do so. Using 40% of all 2000 would answer a different question.

Survivors = Efs; with conditional mortality m, survivors = Ef(1−m).

NCERT Trap: External fertilisation is about fusion location; indirect development is about a larva. The two labels are not synonyms.

Loss versus survival: If fraction m of the fertilised group dies before a stage, fraction 1−m survives. Then survivors = E × f × (1−m). Distinguish mortality within a fertilised group from failure to fertilise in the original clutch.

NEET Method: Write the stage sequence and carry the changing denominator forward after each percentage.

Structural Organisation in Animals

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