Infographic "Visualizing Development: One Baby or More?"

This infographic describes the process by which monozygotic or dizygotic twins are formed. The feature begins with the following introductory text: Humans usually have one baby at a time, but sometimes twins are born. Most often they are from two ova fertilized by two sperm (lower left), resulting in dizygotic twins. Sometimes, however, one zygote splits in two (lower right), resulting in monozygotic twins; if each of these zygotes splits again, the result is monozygotic quadruplets.

The center of the image shows the normal non-twin process of the formation of a baby. It is marked "usually" and shows a zygote with 46 chromosomes. Hours later it has become two identical cells. Months later it forms into a baby (image shown). And finally, there is a picture of a newborn child marked "one baby with 46 chromosomes."

The left side of the image shows how dizygotic twins are formed. Above this side is a Female symbol with 46 in the circle and the words Woman's chromosomes. There are several ova marked "23" (for chromosomes) and beneath it says "sometimes dizygotic twins." Several sperm cells are depicted, and are marked "23" (for chromosomes). Two of the ova are fertilized by two of the sperm cells. Two zygotes are formed, each with 46 chromosomes. Hours later, they split into two cells. It shows "months later" as two images of prenatal babies, and "months later," two newborn babies. Beneath this is a picture of fraternal twins, a brother and sister.

The right side of the image shows how monozygotic twins are formed. Above this side is a Male symbol with 46 in the circle and the words Man's chromosomes. There are several sperms marked "23" (for chromosomes) and beneath it says "sometimes dizygotic twins." The two-celled zygote from the center (or normal) birth pattern splits into two by way of a dotted line with an arrow. The words "one zygote splits into two" are shown. The zygote splits into two images of prenatal babies (months later), and beneath this, a photo shows two twin babies looking quite identical.