Broadly speaking, the fertilization process is the means through which life is reproduced, or is at least the very first step of that larger phenomenon. Life on earth is generally made up of two distinct parts: the genetic material of both a male and a female parent. This is true for animals as well as plants. The biological term for the union of those materials is fertilization. In humans and most other mammals, fertilization happens through sexual intercourse, when the male sperm meets the female egg. In reptiles, birds, and fish, it also usually involves sperm and egg, through the intersection and actual process of fertilization isn’t usually as streamlined. Sometimes the female lays the eggs long before the male comes to fertilize them, or she may receive fertilization through an actual act of copulation. Plant fertilization usually happens when pollen and other materials from the male part of a plant is placed in a female part, often with the help of bees or other insects.
In Humans and Other Mammals
In humans and most other mammals, fertilization most often occurs when sexual intercourse takes place during a female’s “fertile” or ovulation period. Different animals have different cycles, with some ovulating only once a year; humans usually ovulate once a month. A sexually mature human female is usually fertile on about the 14th day of her menstrual cycle. A few days before the woman ovulates, her cervix secretes mucus, which allows sperm to travel faster towards the uterus and into the fallopian tubes. During ovulation, a mature egg is released by the ovary to the fallopian tube. For about 12 to 24 hours, the mature egg is ready to be fertilized.
Sperm released inside the vagina travel towards the uterus to the fallopian tube to seek out the egg. Hundreds of thousands of sperm may be released during ejaculation, but only one gets to penetrate the egg and start the process of process. Human sperm are capable of staying alive for 48 to 72 hours inside the female reproductive tract, and can fertilize the egg as soon as ovulation takes place. When sperm and egg meets, a zygote is formed.
Zygotic Development
The zygote then undergoes cell division and becomes an embryo. Within five to seven days, the embryo is implanted in the uterus. After implantation, the embryo will undergo many stages of development, which, at least in the case of humans, are completed within about nine months inside the womb. During pregnancy, the female body releases a specific hormone called the human chorionic gonadotrophin hormone (hCG), which can be detected in the urine and blood. A positive home pregnancy test after a missed menstrual period usually indicates that fertilization was successful.
Artificial Insemination
The fertilization process can also occur outside the womb, through in vitro fertilization (IVF). Scientists usually only perform this procedure when couples aren’t able to conceive naturally, though is it also sometimes used by zookeepers and other animal handlers in order to breed animals that are kept in captivity. In these situations, traditional means of fertilization may not be possible if the male and female animals live far away, or if they might be traumatized by the changes needed to mate naturally.
IVF is typically done in a laboratory, where mature eggs harvested from the ovaries of the female and sperm from the male are placed in an enclosed dish for fertilization. Any embryos that result are then implanted in the womb of the woman and allowed to develop and grow naturally. Babies born from this process are sometimes called “test tube babies.”
In Non-Mammals
There are a couple of different ways in which reptiles, birds, and other non-mammals like fish reproduce. In general these creatures lay eggs rather than birthing live babies. Eggs are sometimes fertilized while they are still inside the mother, but they can also be fertilized later; a lot depends on the animal and the circumstances. In most cases, though, embryonic development and growth from this point is more or less analogous to humans, although it’s inside an egg rather than in a womb.
In Plants
The plant fertilization process is often strikingly different. Not only is there no sperm and no egg, there isn’t usually an identifiable “baby,” either. Plants do have genetic material that is decidedly both male and female, though, which must join together in order for reproduction to happen. Many plants have both of these parts. As a result, flowers, trees, and shrubs aren’t usually male or female, they’re often both male and female.
Fertilization in these cases almost always involves the transfer of pollen from the stamen, or male part of the plant, to the stigma, or female part; it is usually then shuttled into the pistil, which is basically the plant’s ovary. The result is usually a fruit, flower, or bud that itself contains a replica of the genetic material — often as a seed or a new configuration of pollen — that can help the process start again in another generation.