---
title: "Fertilisation and the First Cleavages"
description: "Fertilisation has to admit exactly one sperm and then start a programme of divisions that add no mass. The blocks against a second sperm and the pattern of those early cleavages already differ between"
canonical: https://lightmysky.com/learn/science/fertilisation-and-the-first-cleavages-mt_9BXQsdP8fI
source: https://lightmysky.com/learn/science/fertilisation-and-the-first-cleavages-mt_9BXQsdP8fI.md
retrieved: 2026-09-12
---

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# Fertilisation and the First Cleavages

Fertilisation has to admit exactly one sperm and then start a programme of divisions that add no mass. The blocks against a second sperm and the pattern of those early cleavages already differ between animal groups, and both differences matter later.

Subject: Science · Area: Organisms & Life Processes · Ages 19 to 20
Page: https://lightmysky.com/learn/science/fertilisation-and-the-first-cleavages-mt_9BXQsdP8fI

## Ready when they can

- Describe the fast and slow blocks to polyspermy and what each one physically does
- Explain why cleavage divisions shorten the cell cycle and skip growth
- Compare two cleavage patterns and relate the difference to yolk

## Lesson: One sperm, then smaller cells

A new animal starts when exactly one sperm joins one egg. Meetings happen in water for frogs and many fish, which release thousands of gametes at once, or inside the mother body for birds, reptiles, and mammals, in a safe moist place. Letting a second sperm in would wreck the plan, so eggs run a fast block and then a slow block against extra sperm.

Once fertilised, the embryo needs a safe place to grow. Mammals guard it inside the body, birds and most reptiles pack it in a shelled egg laid on land, and some sharks and snakes hold the eggs inside until just before hatching. Each plan trades parental cost against survival: guarding inside costs more, but far more young survive.

Then comes cleavage: a run of divisions that skip growth, so each round makes smaller cells, not a bigger embryo. The cell cycle shortens to divide, divide, divide. Yolk reshapes the pattern: heavily yolked eggs cleave differently from eggs with little yolk.

**Tip.** Read any life history as two bargains: one sperm only, then safety for numbers. External release buys numbers cheap, internal care buys survival dear.

**Recap.** Admit one sperm, house the embryo, then divide without growing.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Meiosis and Why Gametes Are Haploid](https://lightmysky.com/learn/science/meiosis-and-why-gametes-are-haploid-mt_hvu0bczcfX)
- [Cell Cycle Checkpoints, Control and Cancer](https://lightmysky.com/learn/science/cell-cycle-checkpoints-control-and-cancer-mt_nwNmCVnZ7O)
- [Human Reproduction](https://lightmysky.com/learn/science/human-reproduction-mt_tQkCzRcWG7)

## Opens up

- [Gastrulation and Laying Down the Body Axes](https://lightmysky.com/learn/science/gastrulation-and-laying-down-the-body-axes-mt_PABfw_4RPs)