From Gametes to a New Animal

Reproduction is only the beginning of producing a new animal. After sperm and egg are produced, they must come together, and the resulting cell must develop into a functioning organism.

In sexually reproducing animals, the general sequence is:

Gametes → fertilization → zygote → embryo → growth and development → juvenile → adult

Different animal groups accomplish these stages in very different ways.

Some release millions of gametes into water. Others fertilize eggs internally. Some embryos develop inside eggs, while others develop inside the parent's body.

These differences are closely related to the animal's environment, lifestyle, and reproductive strategy.

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Fertilization

Fertilization is the fusion of the nuclei of male and female gametes.

In most animals:

• The male gamete is the sperm.
• The female gamete is the egg.

Each gamete contains one set of chromosomes.

When their nuclei fuse, they form a zygote containing genetic information from both parents.

Sperm + egg → zygote

The zygote is the first cell of the new individual.

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What Happens During Fertilization?

Although the details vary between animal groups, fertilization generally involves several important events.

A sperm reaches an egg.

The sperm interacts with and penetrates the egg's outer layers.

The cell membranes of the gametes fuse.

The sperm nucleus enters the egg.

The genetic material from the two gametes combines.

Mechanisms then normally prevent additional sperm from fertilizing the same egg.

The resulting zygote can then begin development.

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External Fertilization

External fertilization occurs when sperm and eggs meet outside the body.

It is particularly common among aquatic animals, including many:

• Fish.
• Amphibians.
• Marine invertebrates.

During a process called spawning, adults may release sperm and eggs into the surrounding water.

Water provides a suitable environment because it prevents gametes from drying out and allows sperm to move toward eggs.

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Challenges of External Fertilization

External fertilization can be inefficient.

Once released, gametes may:

• Be dispersed by currents.
• Be eaten.
• Fail to encounter another gamete.
• Be exposed to unsuitable temperatures.
• Be damaged by changing environmental conditions.

Animals using external fertilization often compensate by producing large numbers of gametes.

If thousands or millions of eggs and sperm are released, the probability that at least some eggs are fertilized increases.

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Timing External Fertilization

Timing is extremely important.

If one animal releases eggs several hours or days before another releases sperm, fertilization may be unlikely.

Many species therefore synchronize spawning.

Environmental signals can include:

• Temperature.
• Day length.
• Tides.
• Rainfall.
• Lunar cycles.
• Chemical signals from other animals.

Large numbers of animals releasing gametes at approximately the same time can greatly increase the probability of successful fertilization.

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Internal Fertilization

Internal fertilization occurs when sperm and egg meet inside the reproductive tract.

It occurs in many:

• Insects.
• Reptiles.
• Birds.
• Mammals.
• Sharks and rays.
• Other animals.

Internal fertilization is particularly useful in terrestrial environments because gametes are protected from drying out.

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Advantages of Internal Fertilization

Internal fertilization provides several potential advantages.

Gametes are:

• Protected from dehydration.
• Less exposed to predators.
• Less likely to be dispersed.
• More likely to encounter one another.

As a result, animals using internal fertilization often do not need to produce as many gametes as animals that release them freely into the environment.

However, internal fertilization usually requires mating and specialised reproductive structures or behaviours.

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Internal and External Fertilization Compared

Neither method guarantees reproductive success. Each is associated with different advantages, costs, and adaptations.

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Development Begins

After fertilization, the zygote begins to divide.

This occurs through mitosis.

One cell becomes two.

Two become four.

Four become eight.

Cell division continues, producing an increasing number of cells.

During these early divisions, the embryo begins the complex process of becoming a multicellular organism.

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From Zygote to Embryo

The early stages of animal development can be simplified as:

Zygote → repeated cell division → multicellular embryo → cell differentiation → tissues and organs

The exact developmental stages and terminology vary among animal groups.

A crucial point is that producing more cells is not enough.

Those cells must also become specialized.

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Cell Differentiation

Differentiation is the process by which cells become specialised for particular functions.

Early embryonic cells eventually give rise to many cell types, including:

• Muscle cells.
• Neurons.
• Blood cells.
• Skin cells.
• Bone cells.

Nearly all these cells contain the same basic genetic information.

They become different because different genes are active in different cells.

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Formation of Tissues and Organs

Specialised cells become organised into tissues.

Tissues form organs.

Organs become integrated into organ systems.

For example:

Muscle cells → muscle tissue → muscular structures → movement

and

Neurons → nervous tissue → brain, spinal cord and nerves → coordination

Development therefore involves increasing levels of biological organisation.

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Embryos Need Resources

An embryo is alive and developing rapidly.

It needs:

• Nutrients.
• Water.
• Oxygen.
• Suitable temperature.
• Protection from physical damage.
• A way of dealing with metabolic wastes.

Different animal groups have evolved different methods of meeting these needs.

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Development in Aquatic Eggs

Many fish and amphibians lay eggs in water.

The developing embryo may obtain nutrients from yolk stored in the egg.

Oxygen can diffuse from the surrounding water.

Waste products can diffuse away.

The surrounding water helps prevent dehydration, but the eggs may remain vulnerable to predators and environmental changes.

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Yolk

Yolk is a nutrient-rich material stored within many animal eggs.

It can provide developing embryos with substances needed for:

• Respiration.
• Growth.
• Formation of new tissues.

Species whose embryos develop for longer periods without receiving nutrients directly from a parent often require substantial stored resources.

The amount and distribution of yolk vary greatly among animal groups.

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Development on Land

Reproduction on land creates a major problem:

How can an embryo remain protected without drying out?

Reptiles, birds, and other amniotes evolved an important solution — the amniotic egg.

An amniotic egg creates a protected environment in which an embryo can develop away from open water.

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The Amniotic Egg

Important structures of a typical amniotic egg include:

• Shell – provides protection while allowing gas exchange.
• Amnion – surrounds the embryo with fluid.
• Yolk sac – contains stored nutrients.
• Chorion – contributes to gas exchange.
• Allantois – involved in waste storage and gas exchange.

These structures help solve several problems associated with development on land.

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Why the Amnion Matters

The amnion forms a fluid-filled environment around the embryo.

This:

• Cushions the embryo.
• Helps protect it from physical shocks.
• Prevents the embryo itself from drying out.
• Provides an aquatic-like environment inside the egg.

This was a major evolutionary development because reproduction no longer had to depend directly on an external body of water.

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The Eggshell

A bird's eggshell must solve two apparently conflicting problems.

It must:

• Protect the embryo.
• Allow gases to move between the embryo and the environment.

The shell contains microscopic pores.

These allow:

• Oxygen to enter.
• Carbon dioxide to leave.

The shell also reduces water loss while still allowing sufficient gas exchange.

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Development Inside the Parent

In many mammals, the embryo develops inside the female reproductive system.

After fertilization and early cell division, the embryo eventually becomes associated with the wall of the uterus.

A specialised organ called the placenta develops.

The placenta allows substances to be exchanged between the maternal and fetal circulatory systems without their blood normally mixing directly.

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The Placenta

The placenta provides an exchange surface.

Useful substances can move toward the developing embryo or fetus, including:

• Oxygen.
• Glucose.
• Amino acids.
• Other nutrients.

Waste products can move in the opposite direction, including:

• Carbon dioxide.
• Urea and other metabolic wastes.

The developing offspring is connected to the placenta through the umbilical cord.

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Adaptations of the Placenta

An effective exchange surface requires efficient transfer of substances.

The placenta has features that support exchange, including:

• Large surface area.
• Thin barriers between maternal and fetal blood supplies.
• Good blood supply.
• Maintenance of concentration gradients.

These features resemble adaptations found at other biological exchange surfaces, such as:

• Alveoli.
• Intestinal villi.
• Fish gills.

Different organs can therefore use similar structural principles to solve similar exchange problems.

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Embryo and Fetus

In mammals, the terms embryo and fetus describe different stages of prenatal development.

During the embryonic stage, major body structures and organ systems begin forming.

During the fetal stage, growth and further development of these structures become increasingly important.

The precise timing differs among species.

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Oviparous Animals

Animals that lay eggs from which offspring later hatch are described as oviparous.

Examples include most:

• Birds.
• Reptiles.
• Amphibians.
• Fish.
• Insects.

Embryos generally rely heavily on nutrients stored within the egg.

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Viviparous Animals

In viviparous animals, offspring develop inside the parent and are born alive.

Most mammals are viviparous.

Some reptiles, fish, and other animals also give birth to live young.

Internal development can provide:

• Protection from predators.
• More stable temperature.
• Protection from dehydration.
• In some species, continuous nutrient supply from the parent.

However, carrying developing offspring requires considerable parental energy.

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Ovoviviparity and Other Strategies

Animal development does not always fit neatly into simple categories.

In some species, eggs are retained inside the parent's body until they hatch or nearly hatch.

The embryo may depend mainly on yolk rather than receiving nutrients through a placenta.

This strategy has traditionally been called ovoviviparity, although modern zoology often uses more precise descriptions because reproductive systems vary considerably among species.

This illustrates an important biological principle:

Reproductive strategies form a continuum rather than a few perfectly separate categories.

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Direct Development

In direct development, young animals emerge looking broadly similar to smaller versions of adults.

They still grow and mature, but there is no dramatically different larval stage.

Examples occur in:

• Mammals.
• Birds.
• Many reptiles.

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Indirect Development

In indirect development, the young pass through a distinct larval stage that differs substantially from the adult.

Examples include:

• Frogs.
• Butterflies.
• Many marine invertebrates.

A major transformation called metamorphosis may occur.

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Metamorphosis

Metamorphosis is a major change in body form during development.

A frog provides a familiar example.

Egg → tadpole → developing frog → adult frog

A tadpole differs considerably from an adult frog.

Tadpoles generally:

• Live entirely in water.
• Swim using a tail.
• Use gills early in development.
• Often feed differently from adults.

During metamorphosis:

• Legs develop.
• Lungs become increasingly important.
• The tail is reduced.
• Feeding structures change.

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Complete Metamorphosis in Insects

Many insects undergo particularly dramatic development.

For example, butterflies undergo:

Egg → larva → pupa → adult

The larva and adult can have very different:

• Body structures.
• Diets.
• Behaviours.
• Habitats.

This can reduce competition between juveniles and adults for the same resources.

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Incomplete Metamorphosis

Other insects undergo incomplete metamorphosis.

A typical sequence is:

Egg → nymph → adult

Examples include grasshoppers and many other insects.

A nymph resembles a smaller version of the adult but lacks some adult structures, such as fully developed wings and reproductive organs.

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Comparing Development in Major Animal Groups

These are general patterns. Exceptions occur within several animal groups.

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Worked Example: Fish

Many fish release eggs and sperm into water.

Fertilization occurs externally.

The embryo develops within an egg.

Stored yolk supplies nutrients.

Oxygen enters from the surrounding water.

After hatching, the young may receive little or no parental care.

Producing many eggs can compensate for high mortality.

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Worked Example: Frog

Many frogs also use external fertilization.

Their eggs are generally laid in water.

The embryo develops into a larval stage called a tadpole.

The tadpole later undergoes metamorphosis.

This strategy allows different life stages to exploit different ecological conditions.

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Worked Example: Bird

Birds use internal fertilization.

The fertilized egg receives nutrients before it is laid.

The embryo develops inside an amniotic egg.

The yolk supplies nutrients.

The shell and membranes provide protection while permitting gas exchange.

Parents commonly incubate the eggs, helping maintain suitable developmental temperatures.

After hatching, many bird species provide extensive parental care.

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Worked Example: Placental Mammal

In a placental mammal:

1. Fertilization occurs internally.
2. The zygote begins dividing.
3. The early embryo travels toward the uterus.
4. Implantation occurs.
5. The placenta develops.
6. Nutrients and oxygen are transferred from the mother.
7. Wastes are transferred away from the developing offspring.
8. Development continues until birth.

This strategy provides considerable protection but requires substantial investment by the mother.

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Development and Parental Investment

Developmental strategy is closely connected to parental investment.

Consider two extremes.

Strategy A

An animal produces thousands of eggs.

Each embryo receives relatively little parental protection.

Most offspring die before reaching adulthood.

A small number survive.

Strategy B

An animal produces one offspring.

The offspring develops internally.

The parent protects and feeds it for a long period.

Its probability of survival may be much higher.

Both strategies can successfully maintain a population.

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Protection Versus Number of Offspring

Animals have limited energy and resources.

A major reproductive trade-off therefore exists:

Many offspring + relatively little investment in each

or

Few offspring + relatively high investment in each

Neither strategy is automatically superior.

Success depends on environmental conditions and the organism's ecology.

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Development in Unpredictable Environments

In environments where juvenile survival is unpredictable, producing many offspring can increase the probability that at least some survive.

For example, aquatic eggs may face:

• Predators.
• Changing temperatures.
• Currents.
• Changing oxygen levels.
• Disease.

Producing many offspring spreads reproductive investment across many individuals.

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Development in Protected Environments

Internal development provides a relatively protected environment.

The developing offspring may experience:

• More stable temperature.
• Reduced dehydration.
• Physical protection.
• Reduced exposure to predators.
• Reliable access to nutrients in placental species.

However, internal development can limit how many offspring can be supported at one time.

It also increases energetic costs for the parent.

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Developmental Timing

The length of development varies greatly among animals.

Factors influencing developmental time can include:

• Body size.
• Temperature.
• Nutrient availability.
• Metabolic rate.
• Developmental strategy.
• Degree of development required before hatching or birth.

Some offspring emerge relatively independent.

Others require prolonged parental care.

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Precocial Young

Precocial young are relatively well developed when they hatch or are born.

They may be able to:

• Walk.
• Follow parents.
• Feed relatively independently.
• Respond quickly to threats.

Examples include young of many ground-nesting birds.

Producing well-developed young requires considerable energy before hatching or birth.

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Altricial Young

Altricial young are relatively underdeveloped at hatching or birth and require substantial parental care.

Many songbirds, for example, hatch:

• Small.
• Relatively helpless.
• Dependent on parents for food.
• Unable to maintain body temperature effectively.

Parents must invest heavily after hatching.

This represents another developmental trade-off:

Greater development before birth/hatching ↔ greater development afterward

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Embryonic Survival

For development to succeed, an embryo must survive several challenges.

It must obtain:

• Energy.
• Raw materials.
• Oxygen.

It must also:

• Remove metabolic wastes.
• Avoid dehydration.
• Avoid physical damage.
• Remain within a suitable temperature range.
• Avoid infection and predation.

Different animal groups have evolved very different structures to solve these same fundamental problems.

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Development and Natural Selection

Developmental strategies are shaped by natural selection.

Characteristics that increase the probability that offspring survive and eventually reproduce can become more common over generations.

For example:

• Protective eggshells can improve survival on land.
• Internal fertilization can prevent gamete dehydration.
• Metamorphosis can reduce competition between life stages.
• Parental care can increase juvenile survival.
• Large numbers of eggs can compensate for high juvenile mortality.

Development is therefore closely connected to an animal's ecological environment.

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Comparing Three Strategies

Consider three animals.

Frog

External fertilization.

Many eggs.

Aquatic embryo.

Aquatic larval stage.

Metamorphosis.

Bird

Internal fertilization.

Relatively few eggs.

Amniotic development.

Protected shell.

Often considerable parental care.

Placental Mammal

Internal fertilization.

Usually relatively few offspring.

Internal development.

Placental nutrient supply.

Often extensive parental care.

Each strategy solves the same fundamental problem — producing surviving offspring — in a different way.

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Worked Example: Why Are Frog Eggs Usually Laid in Water?

Frog eggs generally lack the protective shell found around bird eggs.

If exposed to dry terrestrial conditions, they can lose water rapidly.

Water:

• Prevents dehydration.
• Supports external fertilization.
• Allows sperm to move.
• Provides an environment for early development.

The aquatic environment is therefore closely connected to both fertilization and development.

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Worked Example: Why Can Birds Reproduce Away from Water?

Birds use internal fertilization.

Their embryos develop within amniotic eggs.

The egg contains:

• Stored nutrients.
• Protective membranes.
• Fluid surrounding the embryo.
• A shell that reduces water loss while allowing gas exchange.

The developing embryo therefore carries its own protected environment.

This greatly reduces dependence on external water for reproduction.

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Worked Example: Why Does a Placenta Improve Embryonic Survival?

A mammalian embryo developing inside the uterus does not need to rely entirely on a fixed amount of stored yolk.

The placenta allows continued exchange with the mother.

As the embryo grows:

Oxygen and nutrients → toward developing offspring

Carbon dioxide and metabolic wastes → away from developing offspring

This allows prolonged internal development and can produce relatively well-developed young at birth.

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Common Mistakes

Confusing Fertilization With Development

Fertilization produces a zygote. Development includes the subsequent processes that transform the zygote into a multicellular organism.

Saying External Fertilization Means External Development

The two ideas are related but different. Fertilization describes where sperm and egg meet; development describes where and how the embryo grows.

Saying All Fish Use External Fertilization

Many do, but some fish use internal fertilization.

Saying All Mammals Have Placentas Like Humans

Placental mammals use highly developed placentas, but mammalian reproductive strategies include important variations, including marsupials and egg-laying monotremes.

Saying the Placenta Mixes Maternal and Fetal Blood

The two blood supplies normally remain separate. Substances are exchanged across specialised tissues.

Thinking an Eggshell Is Completely Sealed

Bird eggshells contain microscopic pores that permit gas exchange.

Saying Yolk Is the Embryo

The yolk is primarily a nutrient supply. The embryo is the developing animal.

Confusing Growth and Differentiation

Growth increases size and cell number. Differentiation produces specialised cell types.

Assuming More Parental Care Is Always Better

Parental care can increase offspring survival but requires considerable time and energy and may reduce the number of offspring that can be produced.

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Check Your Understanding

1. Define fertilization.

2. Distinguish between internal and external fertilization.

3. Explain why external fertilization is especially common in aquatic environments.

4. Why do many animals using external fertilization release large numbers of gametes?

5. Describe the sequence from fertilization to the formation of a multicellular embryo.

6. Explain the importance of cell differentiation during development.

7. What resources does a developing embryo require?

8. Explain the role of yolk.

9. Describe three ways an amniotic egg protects or supports a developing embryo.

10. Explain how gases can be exchanged through a bird's egg.

11. Describe the role of the placenta.

12. Why do maternal and fetal circulatory systems need to be positioned close together in the placenta?

13. Distinguish between direct and indirect development.

14. Explain one potential advantage of metamorphosis.

15. Compare the developmental strategies of a frog, a bird, and a placental mammal.

16. Explain how developmental strategy can influence the probability that an offspring survives to adulthood.

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Key Terms

• Fertilization – fusion of male and female gamete nuclei.
• Internal fertilization – fertilization occurring inside the reproductive tract.
• External fertilization – fertilization occurring outside the body.
• Gamete – specialised reproductive cell.
• Sperm – male gamete.
• Egg – female gamete.
• Zygote – first cell formed following fertilization.
• Embryo – early developing multicellular organism.
• Fetus – later stage of prenatal development in mammals.
• Mitosis – cell division producing genetically similar daughter cells.
• Differentiation – process by which cells become specialised.
• Yolk – nutrient-rich material that supplies a developing embryo.
• Amnion – membrane enclosing a fluid-filled environment around an embryo.
• Amniotic egg – egg containing specialised membranes that support development on land.
• Placenta – organ allowing exchange between maternal and developing offspring's systems.
• Umbilical cord – structure connecting a developing mammal to the placenta.
• Oviparous – producing offspring by laying eggs.
• Viviparous – producing live young after internal development.
• Metamorphosis – major change in body form during development.
• Larva – immature developmental stage that differs substantially from the adult.
• Pupa – developmental stage between larva and adult in insects undergoing complete metamorphosis.
• Nymph – immature stage of an insect undergoing incomplete metamorphosis.
• Precocial – relatively well developed at hatching or birth.
• Altricial – relatively underdeveloped and dependent at hatching or birth.
• Parental investment – resources, time, and energy devoted to producing and supporting offspring.

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Key Takeaways

• Fertilization occurs when male and female gamete nuclei combine to produce a zygote.
• Fertilization can occur internally or externally.
• External fertilization is especially common in aquatic environments and often involves the production of many gametes.
• Internal fertilization protects gametes and is particularly important for reproduction on land.
• After fertilization, repeated mitosis produces a multicellular embryo.
• Differentiation produces specialised cells that eventually form tissues, organs, and organ systems.
• Every developing embryo requires nutrients, oxygen, suitable conditions, waste removal, and protection.
• Yolk provides stored nutrients for embryos developing in many types of eggs.
• The amniotic egg provides a protected environment for development on land.
• Bird eggs combine protection, nutrient storage, reduced water loss, and gas exchange.
• In placental mammals, the placenta allows exchange of oxygen, nutrients, and metabolic wastes.
• Direct development produces young broadly similar in body form to adults.
• Indirect development involves a distinct larval stage and often metamorphosis.
• Different life stages can occupy different ecological niches, reducing competition between juveniles and adults.
• Some animals produce many relatively unprotected offspring, while others produce fewer offspring with greater protection and parental investment.
• Developmental strategies involve trade-offs between offspring number, protection, energy investment, and survival probability.
• Fertilization and development are closely adapted to an animal's environment and way of life.