Heredity
Heredity is the transmission from one generation to the next of factors that determine the traits of offspring. Although successful breeding of plants and animals was practiced by humans long before modern civilizations were established, there is no evidence that these early people understood the nature of hereditary factors or how they are transmitted through reproduction.

EARLY HISTORY

One of the early Greek philosophers, Pythagoras (582-509 BC), postulated that all traits of an offspring are derived solely from its father's semen. Aristotle thought that females also produce semen and that the embryo is formed by a fusion in the uterus of both types of semen. He further postulated that both male and female semen are produced by the body's blood.
Leeuwenhoek

Until the 17th century, European medical schools taught that hereditary factors in the semen were derived from vapors emanating from each body organ. However, Anton van LEEUWENHOEK observed human semen through his microscope and reported finding "animalcules." It became generally accepted that sperm were the actual carriers of hereditary factors from males to their offspring. Other biologists studied the ovaries of animals, noted the presence of swollen bodies--which they correctly assumed contained eggs--and hypothesized that these eggs were also units of transmission of hereditary factors.

Epigenesis

Some biologists of the 17th and 18th centuries believed that they saw miniature individuals in the sperm or eggs of various organisms, an observation that led to the doctrine of preformation. According to this theory all parts of the adult are already formed at the beginning of embryonic life, and as a result, embryonic development consists solely of growth. Toward the end of the 18th century, Caspar Friedrich WOLFF conducted extensive investigations on developing chicken embryos.