Reproduction in Animals

Zygote — the single cell formed right after fertilisation (sperm and ovum unite). Embryo — the growing cluster of cells (body is being formed). Foetus — when body parts (hands, legs, heart) become visible — up to about 8 to 9 months. After that the baby is born.

Internal — gametes meet inside the body: humans, cow, dog, hen — higher chance of survival. External — both gametes are released into water: frog, fish — thousands of eggs are laid, not all survive (predators eat them).

The hen has internal fertilisation (the zygote is formed inside), but then it wraps the zygote in layers (yolk, albumen, shell) and lays it outside — the embryo develops inside the egg — the mother sits on it providing warmth (21 days) — the chick hatches. The baby develops inside the egg; it is simply kept outside the body.

(1) Yolk — yellow part — food for the developing embryo; (2) Albumen — white part — provides water and acts as a protective shield; (3) Shell — outer covering — protection and has tiny pores for air. With the mother's warmth, after 21 days — the chick hatches.

The change of form from a larva (immature stage) to an adult is called metamorphosis. Examples: frog — tadpole (has a tail) → frog (tail gone, legs developed); silkworm — larva (produces silk) → pupa → moth. The entire body changes completely.

IVF (in vitro fertilisation): if the mother's oviduct is blocked or the sperm count is low — the doctor collects the ovum from the ovaries, fertilises it with sperm in a laboratory dish (fertilisation happens outside the body) — an embryo is formed — it is placed back into the mother's uterus — the baby is born normally. The name 'test tube' comes from the laboratory dish used — the baby actually grows inside the mother's womb.