---
title: "How Animals Grow Up"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 8
exercises: 10
source: https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up
---

# Chapter 8 — How Animals Grow Up

A kitten looks like a small cat from its very first day. But the frog in the pond began as a fish-shaped tadpole with no [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), and the butterfly on the buddleia spent weeks as a crawling [caterpillar](#ex-g2-animals-grow-up-butterfly). Young [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) do not all resemble their parents — some must transform completely on the way to adulthood.

## 8.1 From the egg or born alive

**Definition 8.1 (Two ways to be born).**

Young [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) come into the world in two ways:

- most hatch from an *egg* laid by the mother: birds, fish, frogs, snails, insects;
- others are *born alive* , coming out of the mother’s body already formed, and drink her *milk* : cats, dogs, cows, rabbits — and humans.

**Example 8.2 (In the henhouse and in the basket).**

The hen lays [eggs](#def-g2-animals-grow-up-born) and sits on them to keep them warm; after three weeks, chicks break the [shell](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and hatch. In the basket by the stove, the cat’s kittens were [born alive](#def-g2-animals-grow-up-born) the same night, and they found her [milk](#def-g2-animals-grow-up-born) within the hour.

**Remark 8.3 (An egg is not a stone).**

Inside a sat-on [egg](#def-g2-animals-grow-up-born), a chick is quietly being built, fed by the yellow yolk. The [egg](#def-g2-animals-grow-up-born) is not waiting — it is working. That is why the hen keeps her [eggs](#def-g2-animals-grow-up-born) warm, and why an [egg](#def-g2-animals-grow-up-born) from the fridge, which was never going to hatch anyway, must not be blamed on the cold.

## 8.2 Growing without transforming

**Definition 8.4 (Growing like the parents).**

Many young [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) are *miniatures of their parents*: the kitten is a small cat, the foal a small horse, the chick a small hen. Growing up, for them, means getting bigger and stronger — the body’s shape stays the same.

**Example 8.5 (Kitten to cat).**

The newborn kitten drinks [milk](#def-g2-animals-grow-up-born) with closed [eyes](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense). At one month it plays and pounces; at six months it hunts like a grown cat; at one year it is an adult, perhaps with kittens of its own. Small cat, bigger cat, cat: the same shape all along the way.

## 8.3 Growing by transforming

**Definition 8.6 (Metamorphosis).**

Some young [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) look nothing like their parents, and must change their whole body shape to become adults. That great change is called *metamorphosis*. The tadpole becomes a frog; the [caterpillar](#ex-g2-animals-grow-up-butterfly) becomes a butterfly.

**Example 8.7 (The frog’s road).**

From the frog’s [eggs](#def-g2-animals-grow-up-born) in the pond hatch *tadpoles*: round-headed, long-tailed, legless, breathing in water like fish. Then, week by week: back [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) appear, then front [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts); the tail shrinks away; and one day a small frog climbs out onto the bank. Same [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) — rebuilt.

![Caught mid-metamorphosis: back legs grown, front legs still to come, tail still long.](https://one-course.com/images/onecourse/chapters/biology-1/g2-animals-grow-up/img-f78de2fb405c.jpg)

*Caught [mid-metamorphosis](#def-g2-animals-grow-up-metamorphosis): back [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) grown, front [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) still to come, tail still long.*

![The metamorphosis of the frog, turning clockwise from the eggs: tadpole, legs, froglet, adult — and the adult lays the next eggs.](https://one-course.com/images/onecourse/chapters/biology-1/g2-animals-grow-up/fig-a7c11606d7d6.svg)

*The [metamorphosis](#def-g2-animals-grow-up-metamorphosis) of the frog, turning clockwise from the [eggs](#def-g2-animals-grow-up-born): tadpole, [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), froglet, adult — and the adult lays the next [eggs](#def-g2-animals-grow-up-born).*

**Example 8.8 (The butterfly’s road).**

From the butterfly’s [egg](#def-g2-animals-grow-up-born) hatches a *caterpillar*, which does little but eat [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) and grow. One day it wraps itself in a case, the *chrysalis*, and stops moving. Inside, the body is rebuilt; after some weeks the case splits, and out climbs a butterfly, unfolding wet new wings. [Egg](#def-g2-animals-grow-up-born), [caterpillar](#ex-g2-animals-grow-up-butterfly), [chrysalis](#ex-g2-animals-grow-up-butterfly), butterfly: four stages of one and the same [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal).

**Method 8.9 (Reading an animal’s life cycle).**

For any [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) you meet:

1. ask how it is born: from an [egg](#def-g2-animals-grow-up-born) , or [born alive](#def-g2-animals-grow-up-born) ?
2. ask what the young looks like: a miniature of its parents, or a different shape?
3. if it is a different shape, it will transform: look for the stages of its [metamorphosis](#def-g2-animals-grow-up-metamorphosis) ;
4. draw the cycle as a wheel, with an arrow from the adult back to the [eggs](#def-g2-animals-grow-up-born) .

**Remark 8.10 (Milk, or straight to work).**

Kittens are fed and washed by their mother for weeks; a newly hatched tadpole gets no help at all and must feed itself from the first day. Some young [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) are cared for, others start life alone — both roads lead to adulthood.

## 8.4 Exercises

**Exercise 8.1 ★.**

Give the two ways young [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) come into the world, with one example of each.

**Solution of Exercise 8.1.**

From an [egg](#def-g2-animals-grow-up-born) laid by the mother (a chick, a frog, a snail), or [born alive](#def-g2-animals-grow-up-born) from the mother’s body and drinking her [milk](#def-g2-animals-grow-up-born) (a kitten, a calf, a human baby).

**Exercise 8.2 ★.**

From an [egg](#def-g2-animals-grow-up-born), or [born alive](#def-g2-animals-grow-up-born): (a) a chick; (b) a kitten; (c) a frog; (d) a rabbit; (e) a snail?

**Solution of Exercise 8.2.**

(a) From an [egg](#def-g2-animals-grow-up-born). (b) [Born alive](#def-g2-animals-grow-up-born). (c) From an [egg](#def-g2-animals-grow-up-born). (d) [Born alive](#def-g2-animals-grow-up-born). (e) From an [egg](#def-g2-animals-grow-up-born).

**Exercise 8.3 ★.**

What does a kitten drink first? Which [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) of this chapter drink the same at [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle)?

**Solution of Exercise 8.3.**

Its mother’s [milk](#def-g2-animals-grow-up-born). All the [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) [born alive](#def-g2-animals-grow-up-born) in this chapter — puppies, calves, baby rabbits, human babies — begin on [milk](#def-g2-animals-grow-up-born) too.

**Exercise 8.4 ★.**

What is [metamorphosis](#def-g2-animals-grow-up-metamorphosis)? Name the two famous examples of this chapter.

**Solution of Exercise 8.4.**

[Metamorphosis](#def-g2-animals-grow-up-metamorphosis) is the great change of body shape some young [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) go through to become adults. The tadpole becoming a frog, and the [caterpillar](#ex-g2-animals-grow-up-butterfly) becoming a butterfly.

**Exercise 8.5 ★.**

Put the frog’s stages in order: frog — [eggs](#def-g2-animals-grow-up-born) — tadpole with [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) — tadpole.

**Solution of Exercise 8.5.**

[Eggs](#def-g2-animals-grow-up-born), tadpole, tadpole with [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), frog.

**Exercise 8.6 ★.**

Put the butterfly’s stages in order: butterfly — [caterpillar](#ex-g2-animals-grow-up-butterfly) — [egg](#def-g2-animals-grow-up-born) — [chrysalis](#ex-g2-animals-grow-up-butterfly). At which stage does the [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) eat [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) and grow the most?

**Solution of Exercise 8.6.**

[Egg](#def-g2-animals-grow-up-born), [caterpillar](#ex-g2-animals-grow-up-butterfly), [chrysalis](#ex-g2-animals-grow-up-butterfly), butterfly. It is the [caterpillar](#ex-g2-animals-grow-up-butterfly) that eats [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) and does nearly all the growing.

**Exercise 8.7 ★.**

Which young [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) is a miniature of its parents: the tadpole, or the foal? What does growing up mean for that one?

**Solution of Exercise 8.7.**

The foal — a small horse from its first day. For it, growing up means getting bigger and stronger while keeping the same shape.

**Exercise 8.8 ★.**

Why does the hen sit on her [eggs](#def-g2-animals-grow-up-born)? What is happening inside them, by [Remark 8.3](#rem-g2-animals-grow-up-egg)?

**Solution of Exercise 8.8.**

To keep them warm. Inside each warm [egg](#def-g2-animals-grow-up-born) a chick is being built, fed by the yolk, until it is ready to break the [shell](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and hatch.

**Exercise 8.9 ★★.**

In the pond, Zoe finds a small [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) with a round head, a long tail, two back [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) and no front [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts). Using the frog cycle figure, say exactly where it is on its road, what it was before, and what comes next.

**Solution of Exercise 8.9.**

It is a tadpole partway through [metamorphosis](#def-g2-animals-grow-up-metamorphosis): the back [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) have grown, the front [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) have not yet appeared. Before, it was a legless tadpole (and before that, an [egg](#def-g2-animals-grow-up-born)); next, front [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) will appear, the tail will shrink, and it will become a small frog.

**Exercise 8.10 ★★.**

“The [caterpillar](#ex-g2-animals-grow-up-butterfly) died in its case, and a butterfly moved in.” What really happened inside the [chrysalis](#ex-g2-animals-grow-up-butterfly)? Answer with [Definition 8.6](#def-g2-animals-grow-up-metamorphosis) and [Example 8.8](#ex-g2-animals-grow-up-butterfly).

**Solution of Exercise 8.10.**

Nothing moved in — the [caterpillar](#ex-g2-animals-grow-up-butterfly) and the butterfly are the same [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal). Inside the [chrysalis](#ex-g2-animals-grow-up-butterfly) the [caterpillar](#ex-g2-animals-grow-up-butterfly)’s body was rebuilt into a butterfly’s: that rebuilding is what [metamorphosis](#def-g2-animals-grow-up-metamorphosis) means.