---
title: "The Reproductive Systems"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 57
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems
---

# Chapter 57 — The Reproductive Systems

[Chapter 54](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#ch-g8-sexual-reproduction) needed [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes); [Chapter 56](https://one-course.com/books/biology/1/en/chapter/56-puberty#ch-g8-puberty) switched their factories on. This chapter maps the factories themselves: the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) and [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) reproductive systems — the organs, the [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) each produces, and the [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) system’s remarkable monthly rhythm. Plain anatomy, plainly named: these are organs like [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) and [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), and this is their chapter.

## 57.1 The male system

**Definition 57.1 (The male reproductive system).**

The [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) system’s organs:

- the two *testes* , held outside the body’s warmth in the *scrotum* — the [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) factories: from [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty) on they produce *sperm cells* continuously, by the hundreds of millions;
- ducts that store and carry the sperm, joined on the way by glands whose fluids feed and carry them — together making *semen* ;
- the *penis* , through which semen [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) the body — the same organ, on its other duty, carrying [urine](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#def-g7-removing-wastes-kidneys) ( [Definition 53.3](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#def-g7-removing-wastes-kidneys) ); a valve arrangement keeps the two duties strictly apart.

**Example 57.2 (The sperm cell, up close).**

Under the [microscope](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-microscope) ([Definition 29.4](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-microscope)), a [sperm cell](#def-g8-reproductive-systems-male) is a [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) stripped for travel: a head that is almost all [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts) — the father’s contribution, packed ([Remark 29.6](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#rem-g5-first-look-at-cells-nucleus)) — and a lashing tail. Smallest [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) in the body, built to swim, provisioned for days. Production runs from [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty) into [old age](https://one-course.com/books/biology/1/en/chapter/15-the-stages-of-life#def-g3-stages-of-life-stages): the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) factory is continuous.

## 57.2 The female system

**Definition 57.3 (The female reproductive system).**

The [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) system’s organs, all within the lower belly:

- the two *ovaries* — the [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) stores and factories: unlike the [testes](#def-g8-reproductive-systems-male) , they hold their stock of future *[egg cells](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)* from [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) , and from [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty) release them one at a time;
- the two *oviducts* , fringed tubes that catch the released [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) — the usual meeting place of [Proposition 54.2](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) ’s step 2;
- the *uterus* (the [womb](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#prop-g5-human-reproduction-plan) of [Proposition 32.1](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#prop-g5-human-reproduction-plan) ), whose thick muscular wall and renewable lining are the [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) ’s lodging;
- the *vagina* , connecting the uterus to the outside — where semen is received, and the [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) canal of [Example 32.6](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#ex-g5-human-reproduction-birth) .

**Example 57.4 (The egg cell, up close).**

The [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) is the sperm’s opposite in everything but role: the body’s *largest* [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — a tenth of a millimetre, at the [eye](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense)’s edge — round, unmoving, stocked with provisions for the journey’s first days (the yolk principle of [Remark 8.3](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#rem-g2-animals-grow-up-egg), at [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) scale). One factory runs continuous millions; the other releases a single provisioned [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) a month: [Proposition 23.7](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#prop-g4-animal-reproduction-strategies)’s numbers-against-care, written into the [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) themselves.

## 57.3 The cycle

**Proposition 57.5 (The menstrual cycle).**

From [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty) to about fifty, the [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) system runs a repeating program of roughly *28 days* — the *menstrual cycle*, ordered by the [hormone](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones) chain of [Definition 56.4](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones):

1. the uterus lining rebuilds itself, thick and blood-rich — the lodging prepared;
2. mid-cycle, around day 14, an [ovary](#def-g8-reproductive-systems-female) releases one [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) — *ovulation* ; the [oviduct](#def-g8-reproductive-systems-female) catches it, and for about a day it can be fertilized;
3. no [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) : the lining is not needed; it breaks down and [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) through the vagina over a few days — the *menstrual period* — and the program restarts;
4. [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) : the [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) settles into the prepared lining ( [Definition 32.3](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) ), the cycle suspends, and [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) begins.

**Proof.** *Admitted at this level.* ∎

![The menstrual cycle as a wheel: the period opens it, the lining rebuilds, ovulation crowns it mid-turn — and fertilization is the one event that stops the wheel.](https://one-course.com/images/onecourse/chapters/biology-1/g8-reproductive-systems/fig-c442f19a4a8d.svg)

*The [menstrual cycle](#prop-g8-reproductive-systems-cycle) as a wheel: the period opens it, the lining rebuilds, [ovulation](#prop-g8-reproductive-systems-cycle) crowns it mid-turn — and [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) is the one event that stops the wheel.*

**Remark 57.6 (Living with the cycle).**

Practicalities, said plainly: cycles are irregular for the first years — the machinery is tuning; length varies from person to person and month to month; periods can bring cramps (the uterus is a [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle), [Definition 17.6](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle), and it works during shedding) — warmth, movement and, when needed, the school nurse’s advice all help; and pads, tampons and cups are simply the period’s practical equipment, a normal aisle of every pharmacy. None of this is illness: the cycle is the system running exactly as built.

**Method 57.7 (Reading the two systems side by side).**

To keep the map straight, compare by function:

1. [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) factory: [testes](#def-g8-reproductive-systems-male) — [ovaries](#def-g8-reproductive-systems-female) ;
2. [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) : continuous swimming millions — one provisioned [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) a month;
3. meeting place: the [oviduct](#def-g8-reproductive-systems-female) , reached by the sperm’s swim from the vagina through the uterus;
4. lodging: none in the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) map — the uterus, monthly prepared, in the [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) ;
5. timetable: continuous from [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty) — cyclic, [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty) to about fifty.

## 57.4 Exercises

**Exercise 57.1 ★.**

Map the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) system: name each organ and its function.

**Solution of Exercise 57.1.**

The [testes](#def-g8-reproductive-systems-male), in the scrotum: continuous sperm production from [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty). The ducts and glands: storage, transport, and the fluids that make semen. The [penis](#def-g8-reproductive-systems-male): semen’s exit — and [urine](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#def-g7-removing-wastes-kidneys)’s, on its separate, valve-guarded duty.

**Exercise 57.2 ★.**

Map the [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) system likewise.

**Solution of Exercise 57.2.**

The [ovaries](#def-g8-reproductive-systems-female): the [egg-cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) stock from [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle), releasing one per cycle from [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty). The [oviducts](#def-g8-reproductive-systems-female): catching the released [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) — the usual site of [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps). The uterus: the muscular, lining-renewed lodging. The vagina: reception of semen, and the [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) canal.

**Exercise 57.3 ★.**

Contrast [sperm cell](#def-g8-reproductive-systems-male) and [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual): size, build, numbers, provisioning.

**Solution of Exercise 57.3.**

[Sperm cell](#def-g8-reproductive-systems-male): the body’s smallest — a nucleus-packed head and a swimming tail, provisioned lightly, made continuously by the hundreds of millions. [Egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual): the body’s largest, round and unmoving, richly provisioned, released one a month.

**Exercise 57.4 ★.**

What happens at [ovulation](#prop-g8-reproductive-systems-cycle), on which rough day, and how long is the [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) fertilizable?

**Solution of Exercise 57.4.**

An [ovary](#def-g8-reproductive-systems-female) releases one [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual), caught by its [oviduct](#def-g8-reproductive-systems-female) — around day 14. It can be fertilized for about a day.

**Exercise 57.5 ★.**

What is the [menstrual period](#prop-g8-reproductive-systems-cycle), in the lining’s terms — and what does its arrival say about that cycle?

**Solution of Exercise 57.5.**

The breakdown and shedding of the uterus’s prepared lining, leaving over a few days. Its arrival says that cycle ended without [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps).

**Exercise 57.6 ★.**

Which event suspends the cycle, and what follows in the uterus?

**Solution of Exercise 57.6.**

[Fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps). The [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) settles into the prepared lining, the cycle suspends, and [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) begins.

**Exercise 57.7 ★★.**

Where do [Proposition 54.2](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core)’s three steps happen on this chapter’s map — organ by organ?

**Solution of Exercise 57.7.**

Step 1, [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes): [testes](#def-g8-reproductive-systems-male) and [ovaries](#def-g8-reproductive-systems-female). Step 2, [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps): in the [oviduct](#def-g8-reproductive-systems-female), reached by the sperm’s swim from vagina through uterus. Step 3, the dividing [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core)’s development: lodged in the uterus’s prepared lining.

**Exercise 57.8 ★★.**

Read the [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) as [Proposition 23.7](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#prop-g4-animal-reproduction-strategies)’s two strategies in miniature. Which side carries which logic?

**Solution of Exercise 57.8.**

The [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) side runs the many-and-cheap logic: continuous swimming millions, each expendable. The [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) side runs few-and-provisioned: one stocked [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) a month, and behind it the uterus’s whole lodging investment. The strategy line, written in [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell).

**Exercise 57.9 ★★.**

Why are the first years of cycles often irregular, and why is a varying length normal? (Two facts from [Remark 57.6](#rem-g8-reproductive-systems-living).)

**Solution of Exercise 57.9.**

The machinery is tuning in its first years — the [hormone](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones) chain settling into rhythm — so irregularity is expected; and cycle length genuinely varies, person to person and month to month. Both are the system running normally.

**Exercise 57.10 ★★.**

Explain period cramps with [Definition 17.6](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) — and name two things that help.

**Solution of Exercise 57.10.**

The uterus is a [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle), and shedding is muscular work — its contractions are felt as cramps. Warmth and movement help (and the school nurse has advice when they are not enough).

**Exercise 57.11 ★★.**

The two factories run opposite timetables — continuous against cyclic-with-stock-from-birth. Connect each to its [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes)’s build and role.

**Solution of Exercise 57.11.**

The sperm’s job is the journey: cheap, swimming, and needed in millions because almost none arrive — so the factory runs continuously. The [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)’s job is to become the [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core)’s first provisions: costly, stocked from [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle), and released with a month’s preparation behind it — so the factory is a treasury, spending one at a time.

**Exercise 57.12 ★★★.**

“The cycle is the uterus asking, every month, the question only [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) can answer.” Unfold the image into the four numbered events of [Proposition 57.5](#prop-g8-reproductive-systems-cycle).

**Solution of Exercise 57.12.**

Event 1 is the question being prepared: the lining rebuilt, the lodging readied. Event 2 poses it: one [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) released — will [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) come? Event 3 is the answer “no”: the unneeded lining sheds, and the wheel asks again next month. Event 4 is the answer “yes”: the [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) settles, the asking stops, and the lodging serves its purpose for nine months.

## 57.5 Problem: The Clinic’s Teaching Models

**Problem 57.1.**

Weekend problem — explaining the systems with two models and a calendar

A health center’s open day uses three teaching props: a male-system model, a female-system model, and a large 28-day cycle calendar. You are the volunteer explainer.

**Part I — The two models.**

1. A visitor traces the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) model from [testes](#def-g8-reproductive-systems-male) to exit. Narrate the route, naming what is added on the way and the double-duty organ’s arrangement.
2. On the [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) model, point out where the [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) stock waits, where [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) usually happens, and where a [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) lodges.
3. A visitor asks why the models’ [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) factories differ so sharply — millions daily against one a month. Answer with [Example 57.4](#ex-g8-reproductive-systems-egg) ’s closing sentence.
4. Another asks what the systems were doing before [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty) . Answer with [Chapter 56](https://one-course.com/books/biology/1/en/chapter/56-puberty#ch-g8-puberty) ’s machinery in one line.

**Part II — The calendar.**

5. Walk the calendar: place the period, the rebuild, [ovulation](#prop-g8-reproductive-systems-cycle) and the fertile day, with their rough dates.
6. A visitor assumes day 28 is the same for everyone. Correct the assumption with [Remark 57.6](#rem-g8-reproductive-systems-living) — two variabilities.
7. Show on the calendar what changes if [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) happens near day 14 — which events are cancelled, which begin ( [Definition 32.3](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) continues the story).
8. A teenager asks, quietly, whether cramps mean something is wrong. Give the [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) answer and the two helps — and the address for anything beyond them.

**Part III — The explainer’s craft.**

9. Visitors giggle at the models. Your opening sentence treats the systems as [Chapter 57](#ch-g8-reproductive-systems) ’s first paragraph does. Write it.
10. Summarize the two systems in [Method 57.7](#met-g8-reproductive-systems-sides) ’s five-line comparison, from memory.
11. A visitor asks where babies’ beginnings fit on the props. Chain the three props into [Proposition 54.2](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) ’s three steps.
12. Close the open day with one sentence on why health centers teach this anatomy at all — knowledge against legend, as [Example 56.8](https://one-course.com/books/biology/1/en/chapter/56-puberty#ex-g8-puberty-questions) argued.

**Solution of Problem 57.1.**

**1.** From the [testes](#def-g8-reproductive-systems-male) — the factories, kept in the scrotum — sperm pass into the ducts for storage and transport, are joined by the glands’ fluids to make semen, and leave through the [penis](#def-g8-reproductive-systems-male); the same organ carries [urine](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#def-g7-removing-wastes-kidneys) at other times, a valve arrangement keeping the two duties strictly apart.

**2.** The stock waits in the [ovaries](#def-g8-reproductive-systems-female), from [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle); [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) usually happens in the [oviducts](#def-g8-reproductive-systems-female); a [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) lodges in the uterus’s prepared lining.

**3.** The [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) are built to opposite strategies: millions of cheap swimmers against one provisioned [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) a month — numbers-against-care, written into the [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) themselves.

**4.** Waiting for their orders: the systems stood built but idle until the brain-base control center’s [hormones](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones) switched them on at [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty).

**5.** Days 1–5 or so: the period. Through mid-cycle: the lining rebuilds. Around day 14: [ovulation](#prop-g8-reproductive-systems-cycle), with the fertile day around it. Then the prepared wait — to day 28 and the wheel’s next turn.

**6.** Cycle length varies from person to person, and month to month in one person — and the first years run irregular while the machinery tunes. The calendar teaches the pattern, not anyone’s exact dates.

**7.** Cancelled: the lining’s breakdown and the next period — the wheel stops. Beginning: the [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core)’s settling into the lining, and [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy)’s nine months.

**8.** Cramps are the muscular uterus working as it sheds — normal, not damage; warmth and movement help, and anything stronger has its address at the nurse’s or doctor’s door.

**9.** For example: “These are organs like [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) and [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) — everyone in this room owns one set or the other, and this is simply their chapter.”

**10.** Factories: [testes](#def-g8-reproductive-systems-male) — [ovaries](#def-g8-reproductive-systems-female). [Gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes): continuous swimming millions — one provisioned [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) a month. Meeting: the [oviduct](#def-g8-reproductive-systems-female). Lodging: none — the monthly-prepared uterus. Timetable: continuous from [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty) — cyclic, [puberty](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-puberty) to about fifty.

**11.** Step 1 on the two models: the factories make the [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes). Step 2 on the [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) model: their joining in the [oviduct](#def-g8-reproductive-systems-female). Step 3 across model and calendar: the [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) lodging in the calendar’s prepared lining, the wheel stopped, development begun.

**12.** “Because bodies run better on knowledge than on legend — and everything taught today has a trained, confidential address for its follow-up questions.”