In the 17th century, it was still unknown that gametes fuse to form a zygote. Aristotle proposed that the sperm forms an egg when inside the female, which then forms an embryo. Harvey thought that animals came from eggs and that semen (provided by sexual intercourse) was needed to cause the egg to form an embryo. Harvey dissected a uterus from a female deer that had recently mated; he could not find an embryo. He only saw embryos in deer that had mated several weeks earlier. He was unable to to gain the evidence he needed to describe the process of gamete fusion. He thought his own ideas were not fully correct. The magnifying equipment that existed at that time was not good enough for him to see sperm, eggs, or early stage embryos (these were too small). The discovery of sperm fusing with eggs and embryo development required the invention of the microscope.

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