The introductory text reads, 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.
A flowchart depicts the fertilization process in humans with illustrations of sperm,
ova, and babies. A woman’s 46 chromosomes are depicted using four ova, 23 chromosomes each. Man’s 46
chromosomes are depicted in eight sperm, each carrying 23 chromosomes. Usually, a sperm fertilizes one ovulated ovum
to form a zygote that bears 46 chromosomes. Hours later, the zygote duplicates into two identical cells. Months
later, the baby forms in the womb. Finally, one baby with 46 chromosomes is born.
In the lower right portion,
the flowchart continues from the step showing the formation of two identical cells from the zygote. Sometimes,
monozygotic (identical) twins are born. One zygote bearing 46 chromosomes splits into two identical cells hours
later that result in two babies in the womb months later. Finally, two identical babies are born.
In the lower
left portion, the flowchart shows that sometimes dizygotic twins are born. Sperms bearing 23 chromosomes penetrated
two ovulated ova to form two zygotes, each bearing 46 chromosomes. Hours later, each zygote divides into two
identical cells, and months later, a baby forms in the womb from each zygote. Finally, two fraternal twins are born.