The understanding of how babies are made was not a sudden revelation by a single person but a centuries-long progression of scientific inquiry. This journey moved from philosophical speculation to direct microscopic observation and experimentation. The question of “who” discovered the process is best answered by exploring the historical milestones and the individuals who incrementally revealed the biological mechanisms of reproduction.
Ancient Views and the Dominant Theory
For millennia, the nature of conception remained a mystery, leading to numerous philosophical and medical theories that pre-dated the microscope. The Greek philosopher Aristotle proposed that the male’s semen provided the “form” or “soul” of the new life. The female’s contribution, primarily menstrual blood, was considered the passive “matter” used to construct the embryo.
The Roman physician Galen later challenged this view, championing a “two-seed” theory. He argued that both the male and the female contributed a form of “semen” or generative fluid to conception. Galen suggested the ovaries were analogous to the male testes, and the mixture of these two seeds formed the new organism. Despite these differences, the “preformationist” idea dominated, suggesting a miniature, fully-formed organism was already present within one of the seeds, needing only to unfold and grow.
The Discovery of the Essential Cells
The breakthrough that moved the field from philosophy to biology required the invention of the microscope, which led to the discovery of the physical components necessary for conception. In 1677, the Dutch scientist Antonie van Leeuwenhoek, a pioneer in microscopy, examined human semen. He reported seeing countless tiny, wriggling forms, which he called “animalcules,” and recognized them as a fundamental part of the male contribution to reproduction.
Van Leeuwenhoek’s discovery of the spermatozoa—the male sex cell—was a turning point. However, he and many others mistakenly believed these animalcules contained the pre-formed miniature human.
The female counterpart, the mammalian ovum or egg cell, proved more elusive due to its small size and internal location. The Dutch anatomist Regnier de Graaf identified the ovarian follicles in the late 17th century, but incorrectly thought the entire follicle was the egg. The true mammalian ovum was definitively identified in 1827 by Karl Ernst von Baer, who found the minute sphere while examining the ovaries of a pregnant dog. Von Baer’s discovery confirmed that all animals, including mammals, develop from an egg, displacing the old concept that the female provided only passive matter.
Proving Fertilization: The Moment of Union
With both the sperm and the ovum identified, the next challenge was understanding how they interacted to create a new life. Early theories suggested the sperm provided a stimulating vapor or fluid to activate the egg. This idea was tested by the Italian scientist Lazzaro Spallanzani in the 18th century through experiments on frogs and toads.
Spallanzani demonstrated that mere contact with the fluid portion of semen was insufficient for fertilization, concluding that a physical contribution from both male and female sources was required. He performed the first recorded successful artificial insemination on a dog. Despite his methods, Spallanzani still incorrectly believed that the solid parts of the semen, excluding the “animalcules,” were the fertilizing elements.
The modern understanding of fertilization at the cellular level was confirmed in 1875 by the German zoologist Oscar Hertwig. Studying the transparent eggs of sea urchins, Hertwig was the first to observe the actual moment of conception. He witnessed the sperm entering the egg and saw the nucleus of the sperm and the nucleus of the egg physically fuse to form a single new nucleus. This observation proved that fertilization was the union of the genetic material from both parents, establishing the cellular basis of inheritance.
Charting the Stages of Development
Once the mechanism of conception was understood, attention shifted to the subsequent stages of fetal growth. The English physician William Harvey, known for his work on blood circulation, contributed to early comparative embryology in the 17th century. Harvey championed the idea that all life comes from an egg, laying the groundwork for later studies.
The concept of epigenesis—the idea that the organism develops gradually from an undifferentiated mass—was confirmed over the old preformation theory. Later, Christian Pander and Karl Ernst von Baer established the concept of “germ layers” in the 19th century. They showed that the embryo first forms distinct layers of tissue—the ectoderm, mesoderm, and endoderm—from which all the organs and structures of the adult body arise. This descriptive phase of embryology provided the framework for understanding how a single fertilized cell transforms into a complex organism.