Study Guide

Animal Reproduction Terms

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Animal Reproduction Terms


A little sac at the head of a sperm. It is filled with enzymes that help the sperm get into an egg, like its own personal safe-cracking kit.


Different variants of a gene, like the King of Hearts and the King of Diamonds.


Special embryonic membrane in amniotic egg. Important for gas exchange.


Special embryonic membrane in an amniotic egg that directly surrounds the embryo/fetus. Filled with amniotic fluid. Makes the womb like a big water balloon.


Birds, reptiles, and mammals. Their eggs require special membranes because they develop on land.


Fish and amphibians. Their eggs develop in the water with only a simple embryonic membrane.

Asexual Reproduction

One parent required to reproduce. Nothing sexy about it.

Biological Imperative

The driving force to reproduce. It is almost like every animal is under a witch's spell to make more animals.

Blocks to Polyspermy

Things that happen to prevent an egg from being fertilized by more than one sperm. The egg doesn't have a bouncer at the door, so it uses other methods to get rid of extra sperm.

Blastocyst (or blastula)

Stage of embryo development where cells form a hollow ball. Almost like somebody blasted out the middle.


Type of asexual reproduction where new animals grow like a blob on a parent and then split off


Special embryonic membrane in amniotes. Surrounds amniotic sac, allantois, and yolk sac. Lies just beneath the egg shell.


A structure housed in the nucleus and composed of DNA and proteins containing part of the genetic information of the cell.


An opening used for both reproduction and waste removal. Found in birds, reptiles, amphibians, and monotremes. You never know what is going to come out of this hole.


The act of physically bringing together the sex organs (for example, penis, cloaca, or vagina) for gamete release. The cops can't stop these gametes from getting together.


Same as copulation.


Finding and wooing a mate.

Courtship Ritual

Special behavior to communicate interest to a potential mate. Special dance moves, tail waggles, sniffing urine, etc.


The characteristic of having two sets of chromosomes. In most cases, with the exception of some plants, all somatic, or body, cells in a sexually reproducing organism are diploid.


One of the germ layers formed in gastrulation. It will form outer things like skin, hair, and teeth. It also forms the nervous system.


One of the germ layers formed in gastrulation. It will form most of the organs.


The female gamete, also called an oocyte. It can also mean the full structure that the oviparous embryo develops in, like a hard bird or reptile egg.

Egg Tooth

A tiny nub of hard deposit usually found on a baby bird's beak. It is used to break out of the egg. It usually falls off after hatching.


The act of a male releasing his sperm.

Estrous cycle

The hormone cycle that controls a female's gamete production. Some mammals have a menstrual cycle instead.

External Fertilization

Union of sperm and egg that occurs outside the body in the environment. Some fish could probably sell tickets to this event.


The fusion of two haploid gametes to make a diploid zygote.


Type of asexual reproduction where the animal splits in half (or sometimes more parts).


The whip-like tail that a sperm has. This powerful engine propels the sperm over huge distances.


Located in the ovary. Each follicle contains a single primary oocyte.


A reproductive cell or germ cell in sexually reproducing organisms. Gametes are haploid: they carry a single set of unpaired chromosomes. In females they are called eggs, and in males, they are called sperm.


A stage of embryo development where the blastocyst differentiates and the cells are not identical anymore.


An early developmental process during which the cells of an embryo move with respect to each other. The embryo begins this stage as a blastula with two types of cells, those on the outside of the ball of cells and those on the inside. Gastrulation involves the some of the outer cells pushing themselves into the ball to form a third layer of cells. (Think about pushing your thumb into an inflated balloon.) Gastrulation forms three germ layers. Watch it happen here.


A functional unit of genetic material that is transcribed and contributes to a heritable trait. A gene is a unit of biological inheritance.


The complete set of genes in a full set of haploid chromosomes in a given organism.

Germ Cells

The pre-gamete cells. Before they were sperm and eggs they were spermatogonia and oogonia.

Germ Layers

The first cells in an embryo that have a destiny. The three layers: ectoderm, endoderm, and mesoderm each have a different future.


The characteristic of having one set of unpaired chromosomes. Gametes (eggs and sperm) in a sexually reproducing organism are haploid.


A group of females controlled by one male, who has exclusive mating rights among those females.


An animal that can make both the male and female gametes. A Jack/Jill of all trades.

Internal Fertilization

Union of the sperm and eggs that takes place inside the mother. How hospitable.

Intersexual selection

Mate selection where males and females are involved in selection. Usually one sex is choosy, and the other sex shows off.

Intrasexual selection

Mate selection where males compete with males (or females compete with females). The other sex is usually just a super awesome amazing prize.


When an animal can reproduce more than once in its lifetime. They are doing "it" a lot.


The process of giving birth in viviparous animals like mammals.

Law of Independent Assortment

Says that during meiosis each chromosome gets sorted into the 4 haploid gametes independently of the other chromosomes. So, a single gamete could have the maternal OR paternal version of chromosome 1 and the maternal OR paternal version of chromosome 2. The manager of giving out versions of chromosome 1 does not consult the manager of chromosome 2.

Law of Independent Segregation

Says that each gamete only gets one version of each chromosome, either the maternal or the paternal version. This is obvious if you remember that gametes are haploid (and remember what haploid is).

Leydig Cells

Cells in the testes which secrete hormones during sperm production. Ooh ley ley.


A special type of mammal that delivers undeveloped young and then carries them in a pouch for further development. Kangaroos and Koala bears are marsupials.


A partner for sexual reproduction.


The process of sharing gametes in sexual reproduction. The conversation probably goes like this: "So, I'll provide the 30 million sperm, and you provide the one egg."


When children eat their (hopefully delicious and nutritious) mother. This has been well-studied in spiders.


A specialized cell division resulting in germ cells or gametes. A diploid cell goes through a round of DNA replication and then two rounds of division, therefore producing four genetically different haploid gametes.

Menstrual Cycle

The cycle of hormones which controls the build-up of the uterine lining. If an egg is not implanted, the endometrium is too sad to continue. It is then excreted from the body.


One of the germ layers. These cells will turn into muscle, blood vessels, and connective tissue.


Only having one mate. The old ball and chain could be around for one mating season or for their entire life.


Special type of mammals that lay eggs, like the platypus.


Stage of embryo development where the solid ball of cells look like a raspberry. This is right before they become a hollow blastocyst.


The female gamete. Also called an egg.


The process of producing the female gametes.


The female organ where oogenesis takes place.


An animal that develops in an egg (and gets nutrition only from the egg), but the egg is inside one of the parents and not in the environment.


An animal that develops in an egg that is out in the environment. Birds, reptiles, most fish, amphibians, and monotremes are oviparous.


When a female shoots an egg out of her ovary.


A virgin birth where the reproduction occurs even though the female egg has not been fertilized.


The process of giving birth. We bet many moms wish they just had an ejector seat button.


The male accessory sex organ used to deposit sperm inside a female for internal fertilization.


Chemicals that are released by some animals, often as a part of a courtship ritual. Mmm, smells like someone is available.


The connection between mother and fetus in mammals through which the fetus gets nutrition and gas and waste exchange.

Polar Body

The smaller cell formed in both the meiotic divisions of oogenesis. Only 1 of the 4 haploid cells created will become an oocyte. The others are polar bodies and will deteriorate.


Having more than one mate.

Primary Oocyte

The oocyte in arrest from birth until puberty. It will eventually undergo the first division of meiosis. Are they lazy or just very patient?

Primitive Streak

A section of cells formed early in gastrulation which makes the egg asymmetrical and provides the first bit or organization during gastrulation.


The process during meiosis where of bits of maternal and paternal chromosomes swap chunks of DNA.


A type of asexual reproduction where an animal can re-grow after part of it is removed. Perhaps vampires heal in a similar way.

Secondary oocyte

The large cell after the first meiotic division of oogenesis that can be fertilized.


When an animal only reproduces one time in its life. Big-bang reproduction.

Seminiferous Tubules

The part of the testes where spermatogenesis party takes place.

Sexual Dimorphism

A difference between males and females. Size, color, strength, etc.

Sexual Reproduction

Reproduction requiring the union of sperm and egg.


Process of fish and amphibians releasing their gametes into the water for external fertilization. Have you ever seen a fish yawning while spawning?


The pre-sperm germ cells. They will become sperm at the end of spermatogenesis.

Spermatozoa (Sperm)

The male gamete.


The process of making sperm. There is a blue ribbon recipe in the Testicular Cookbook.

Testicles (Testes)

The male sex organ where sperm are produced.

Umbilical cord

The cord that plugs the fetus into the placenta. Have you ever heard of cutting the cord? Yep, this is the one.


A female orifice only used for reproduction.


When animals give live birth. Mammals are viviparous.


A source of nutrition inside an egg. The yellow part inside a chicken egg is the yolk.


A fertilized egg.

Roots of Common Terms in Animal Reproduction


G = Greek, L = Latin


blast = germ, embryo, bud (G); cyst = capsule (G)


dipl= double (G); oid= like (G)


ecto = outside (G); derma = skin (G)


trus = thrust


fertilis = fruitful (L);


fiss = split (L)


flagrum = whip (L)


gaster = stomach (G)


gest = to bear or carry (L)


hapl = single (G); oid = like (G)


matr = mother (L); phag=to eat (G)


mon = one (G); trema = hole (G)


oon = egg (G); kytos = hollow, receptacle, basket (G)


ov = egg (L); par = near (G);


parthenos = virgin (G); genesis = creation (G)


pens = hang (L)


phero = bring (G); hormo = to set in motion (G)


plac = plate or tablet (G)


poly = many(G); gamos = marriage(G)


re = again (L); pro = forward (L); duct = lead (L)


sperm = seed (L); zoion = a living being (G)

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