Aristotle (384-322BCE) who did work on developing chick eggs, described the two principally possible routes to development, preformation and epigenesis. Aristotle favored epigenesis, ie, when the embryo starts as an undifferentiated mass and parts are added in a specified order. Preformation was however also became popular and stated that either the male or the female provided a miniature individual.

Aristotle, who was rather patriarchal, thought that the semen provided the "form" or the soul and that the first part that developed was the heart.

William Harvey (1578-1657) began working on embryology inspired by his teacher Girolamo Fabrici (c:a1533-1619) and set out to confirm Aristotle's epigenetic theory. His book On the Generation of Animals was published 1651 and contained data on deers and chicks. He was not able to see any embryo in a deer until after six or seven weeks after mating. He did confirm epigenesis but many of his followers turned to preformation.

Antonie van Leeuwenhoek (1632-1723) who improved the microscope and first saw microorganisms indeed discovered spermatozoa in 1677.

In 1759 Friedrich Wolff (1733-1794) published a celebrated treatise Theory of Generation. He claimed that organs were not preformed but were added step by step. The mammalian egg was discovered during the 19th century, 1826, by Karl Ernst von Baer (1792-1876). Together with Heinz Christian Pander and based on Wolff's work he described the three germ layers ectoderm, entoderm and mesoderm. The human egg was not discovered until 1928.

Von Baer was born in present day Estonia and educated in Tallin and at the University of Tartu. He also studied in Berlin and Vienna. In 1817 he became professor in Köningsberg University and full professor of zoology in 1821 and in anatomy in 1826. In old age he unfortunately diligently argued against Darwin's evolutionary theory.

In the second half of the 19th century microscopes allowed cell nuclei to be visualized and it was seen that nuclei from sperm and egg fused after fertilization. This ended the idea that sperm stimulated the egg by physical or chemical means. It was a German botanist Nathanael Pringsheim (1823-1894) who was among the first to study sperm egg fusion in freshwater algae in the 1850s. At about the same time DNA was discovered in 1869 by Swiss biochemist Friedrich Miescher (1844-1895) but the finding was dismissed as unimportant. Chromosomes were also found. All this time Gregor Mendel's results from 1865 lay dormant.

Data from Wikipedia and from here and here.

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