The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology (2015)
Antonie van Leeuwenhoek (1632–1723), Lazzaro Spallanzani (1729–1799), Oscar Hertwig (1849–1922)
During the seventeenth and eighteenth centuries, one of the great scientific questions of philosophical and religious interest was that of procreation, in particular, that of humans. Some argued that the egg was the seed that gave rise to the animal, while others believed it was semen. The nature of semen when impregnating the egg was perceived to be ethereal and variously described as a spirit, a vapor, or an odor, but not physical. In 1677, the Dutch microscopist Antonie van Leeuwenhoek examined semen from various animal species, as well as his own—acquired not by sinfully “abusing” himself, he asserted, but from conjugal coitus—and found many spermatozoa, but at that time did not associate them with impregnation. However, in 1683, he concluded that “man comes not from an egg but from an animalcule in the masculine seed” and that parts of the egg were transferred to the spermatozoon.
The Italian priest-biologist Lazzaro Spallanzani accepted the preformation theory, which asserted that all living beings were created by God at the beginning and were encapsulated within the first female of their species. The new individual within the egg was preformed and, under the influence of semen, expanded. In 1768, Spallanzani was first to describe that both the solid fraction of semen and the egg were essential for reproduction. However, he failed to recognize the role of spermatozoa in the reproductive process.
In the 1870s, there were two views regarding the process of fertilization: the spermatozoa made contact with the egg and, by transmitting a mechanical vibration, stimulated its development; alternatively, the spermatozoa physically penetrated the egg and mixed its chemical components with the egg yolk. In 1876, Oscar Hertwig, a German embryologist, sought to study this problem using the sea urchin, because it was transparent, had a finely divided yolk, and lacked a membrane. He was able to microscopically visualize spermatozoa as they entered the egg and fused with its nucleus. Moreover, Hertwig observed that only a single spermatozoon was required to fertilize the egg, and that once the spermatozoon (sperm) entered the egg, a membrane barrier was formed precluding penetration by additional spermatozoa.
SEE ALSO: Leeuwenhoek’s Microscopic World (1674), Theories of Germination (1759), Germ-Layer Theory of Development (1828), Meiosis (1876).
In the mid-eighteenth century, Spallanzani proposed that all humans were encapsulated within the first female (Eve) and expanded under the influence of semen. This 1913 statue, Eve and the Serpent, by the Belgian sculptor Albert Desenfans (1845–1938) is located in Josaphat Park in Brussels.