RICHARD G. RAWLINS / CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY RICHARD G. RAWLINS / CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY
Fertilised human egg. Digitised light micrograph showing a recently fertilised egg (ovum, within an orange coat). Inside the ovum, the male and female pronuclei (two pale red circles next to black granular area) are seen undergoing fusion. Following the moment of fertilisation and the successful penetration of the ovum by a single sperm, the genetic material of the sperm and ovum combines. The 23 chromosomes of the sperm pair up with the 23 chromosomes of the ovum to make up the full 46 chromosomes that is found in each human cell. The fertilised ovum is known as a zygote.
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