Fertilization is the union and fusion of gametes to initiate the formation and development of a new life. It is also referred to as:

  • Conception
  • Fecundation
  • Syngamy
  • Impregnation


Types of fertilization process

Fertilization depends on the specie of a certain being, below are the different types of processes:

Internal fertilization

  • Humans – internal fertilization involves introducing the sperm into the female reproductive tract.
  • Mammals, reptiles, fishes – the penis [or other forms of intromittent organ] is being introduced into the vagina.
  • Birds – in some birds, internal fertilization is done via a cloacal kiss, where 2 animals press their cloacae together to transfer the sperm.
  • Other animal species – in salamanders, spiders, insects, mollusks, undergo internal fertilization via a spermosphere (collected sperms).

Advantages of internal fertilization

  • Minimal remains of gametes
  • Higher chances of individual egg fertilization
  • Selective fertilization
  • Longer duration of egg protection

External fertilization

External fertilization is the union of gametes outside the body of the parents’ organs. Below are the different types of externa fertilization:

Pregnancy week by week
  • In vitro fertilization (IVF) – is the process where egg cells are fertilized by the sperm outside the womb/uterus
  • Broadcast spawning – the releasing of gametes into the water
  • External ejaculation without intercourse

Advantages of external fertilization

  • Minimal or no contact of bodily fluids
  • Minimal or no transmission of bodily fluids thus, lowering the risk of disease transmission
  • Larger genetic variation

Cross – fertilization

Cross – fertilization (also known as allogamy) is the fertilization of an ovum from an individual carrying another’s male gamete.

Advantages of cross-fertilization

Charles Darwin’s 1876 book suggests that cross – fertilization resolves the conundrum of breeding depression. With that being said, here are the following benefits:

  • Immense height booster
  • Immense weight gain
  • Constitutional stamina
  • Potency over the self – fertilized offspring from one of the same parents

Outcrossing fertilization

Outcrossing fertilization (also called as outbreeding), is a type of mating which involves introducing an unrelated genetic organism [either animal or plant] into the same species or breeding line.

Advantages of outcrossing fertilization

  • Increases genetic variability (also known as avoidance of extinction)
  • Increases genetic range
  • Reduces the risks of genetic abnormalities
  • Lowers the instances of a genetic organism being affected with various diseases
  • Upholds adaptation

Self – fertilization

Self – fertilization (also known as autogamy) happens in hermaphroditic organisms [such as plants and flatworms] where 2 gametes from a single individual fuse together.

Genetic recombination

Genetic recombination (also called as DNA recombination) is the production of a new organism from a combination of traits in contrast exchange of genetic materials between multiple chromosomes.

Human fertilization

Did you know that scientists only discovered the process of human fertilization during the 19th century? Human fertilization is the union of sperm and ovum which initiates the formation of a new human being. That said, here is how fertilization takes place:

Process of human fertilization

  1. Usually copulation (commonly known as sexual intercourse) takes place which enables a man to ejaculate semen [release of 100 – 300 million sperm cells] in the vagina. In line with this, the liquid part of the semen nourishes the sperm. It also coagulates the sperm as it is being awash by the cervical mucus.
  2. As the sperm journeys through the cervix, it will undergo through capacitation for several hours to strengthen its motility and weaken its membrane [since a freshly ejaculated sperm is still incapable of fertilizing oocytes]
  3. Followed by ovulation – release of an egg cell from the ovaries and moves down the ampulla of a fallopian tube.
  4. The sperm bores through the corona radiata (outer layer of follicle cells outside the secondary oocyte), then the cell membranes of each will fuse. This then allows the sperms to penetrate through the ovum’s zona pellucida (outer layer of the egg’s membrane) and its head will bind to a ZP3 glycoprotein. Once meeting the secondary oocyte, the binding triggers the acrosome [of the sperm] to react, producing enzymes to help the sperm get through the zona.
  5. Once the sperm passes trhough the zona pellucida, cortical reaction takes place. The cortical granules [found inside the secondary oocyte] fuse with the cell’s plasma membrane, expelling the granules from the exocytosis to the zona pellucida.
  6. Glycol – proteins cross – link with each other and the enzymes cause the ZP2 to hydrolise into ZP2f. This in turn blocks other sperm from fertilizing the same egg.
  7. The spermatozoan pierces the ooplasm which will be drawn out into a conical elevation (also called as cone of attraction or reception cone). Once entered, the outer portion of the ooplasm transforms into a perivitelline membrane, which blocks other spermatozoans from entering.
  8. The sperm penetrates the ovocyte (cytoplasm of the oocyte), then the oocyte undergoes 2nd meiotic division producing the haploid ovum (immature egg cell) and releasing a polar body. Once the division is completed, the haploid ovum matures.
  9. The sperm’s tail and mitochondria disengage with the formation of the male pronucleus (oocyte’s nucleus).
  10. Then fertilization takes place, the nucleus of the succeeding sperm fuses the ovum to form a diploid cell (also known as the zygote). The chromosomes carried by the gametes starts to bind to fertilize the ovum. The successful fusion of gametes forms a zygote, which will be the soon – to – be baby.
  11. After a few hours, the zygote divides for how many times.
  12. After a week, the blastocyst (mass of 100 cells, which will have 3 germ layers namely endoderm, ectoderm, and mesoderm) reaches the uterus and implants itself into the uterine lining.

Other types of human fertilization

  • Artificial insemination
  • Intercourse right after ovulation

Human fertilization age

Fertilization age is the endpoint of the embryo or fetal development. It is also referred to as:

  • Resultant age
  • Fertilization age
  • Fertilizational age
  • Embryonic age
  • Fetal age
  • Intrauterine developmental age (IUD)

Animal fertilization

Did you know that scientists studied mice and sea urchins to discover the dynamics of their fertilization? The research found out how their sperm and egg meet, and how only a single sperm gets into the egg.

With that being said, here are the 3 steps of fertilization that ensures biological specificity:

  • Chemotaxis – is a type of ligand/receptor interaction which helps the sperm to find an egg with the aid of a Resact (a 14 amino acid peptide purified from a punctulata’s coat) to attract the movement of sperm
  • Acrosomal reaction (commonly known as sperm activation)
  • Sperm and egg adhesion

Type of internal fertilization in animals

Oviparous animals

Oviparous animals are the breeding lines that reproduce offspring by laying eggs such as the following below:

  • Birds
  • Amphibians
  • Reptiles
  • Monotremes
  • Most insects
  • Some fishes
  • Some mollusks
  • Some arachnids

Ovoviviparous animals

Ovoviviparous animals are the breeding lines that use internal fertilization with no placental connection. That said, below are the following ovoviviparous animals:

  • Sharks
  • Rays

Viviparous animals

Viviparous animals are the breeding lines that reproduce live young that are developed and nourished inside the mother’s body.

Types of external fertilization in animals


Amplexus is a breeding behavior where the male holds the female with his anterior legs. As part of the process, the male fertilizes the female’s eggs at the same time (or with some time delay).

That said, below are the following amplexus animals (amplectant pairs):

  • Horseshoe crabs
  • Toads
  • Frogs

Other types of fertilization

Aside from the process of human fertilization, the dynamics of other types of fertilization has also been studied:

Plant fertilization

Non – vascular plants

Non – vascular plants refers to embryophytes (land plants) that do not a vascular system consisting the xylem and phloem. Plants belonging in this category do not have roots, stems, or leaves.

These types of plants do not have true tissues, though some have specialized tissues for internal water transport.

Vascular plants

Vascular plants refers to the ones that reproduce via the use of spores, these consists stems, leaves and specialized non – lignified tissues to conduct the products of photosynthesis. That said, below are the following vascular plants:

  • Ferns
  • Clubmosses
  • Horsetails


Gymnosperm comes from the Greek words gymnos, which means naked and sperma, which means seed. Gymnosperms refer to the group of seed – producing plants which develop either on on the scales’ surface or leaves [habitually modified to form cones].

That said below are the following gymnosperms:

  • Conifers
  • Cycads
  • Ginkgo biloba (habitually modified at the edge of its short stalks)
  • Gnetales

Flowering plants

Flowering plants are the ones that are enclosed with an ovary, they are considered as the most diverse embryophytes. Flowering plants are also referred to as:

  • Angiosperms or angiospermae
  • Magnoliophyta

Double fertilization

Double fertilization is a complex fertilization process which involves the fusion of a female gametophyte (embryo sac) with 2 male gametes.

Of the 2 male gametes, one sperm fertilizes the gametophyte resulting to a diploid zygote. Then the other sperm fuses with the 2 central – cell maternal nuclei (haploid polar nuclei) in the center of the gametophyte, resulting to the development of the triploid cell in the endosperm.

Self – pollination

Self – pollination is when the pollen from the same plant lands at the flower’s stigma (flowering plants) and/or at the ovule (gymnosperms). In line with this, self – pollination brings a lot of advantages:

  • Reproductive success
  • Improves colonization capability

Insect fertilization

Delayed fertilization

  • Odonata (such as dragonflies and damselflies) – is a breeding behavior where females [may] mate with multiple males and store their sperms until the eggs are laid. The male may also do the oviposition (male hovering on the female during egg – laying) to hinder the female from mating with other males and replacing his gametes
  • Hymenoptera (such as ants, bees, wasps) – is a breeding behavior where the pair flies while doing the deed

Protozoa fertilization

The processes of protozoa (also referred to as protists) fertilization are characterized in 3 types:


Gametogamy is the union of 2 single – celled gametes to form a zygote.


Gamontogamy refers to the combination of gamonts to produce agamonts.