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Aristotle thought that fluids from the male and female come together and somehow combine, so that form and function emerge only gradually in a way he called epigenetic. The idea of a process of fertilization required first the idea of an egg, so that there was something to be fertilized. Karl Ernst von Baer played an important role here when he observed a mammalian egg for the first time. Chick and frog eggs were big and obvious, but it wasn’t clear whether all organisms had eggs or not. Von Baer thought they must and went looking, offering the first clear description in 1827 with an egg from a dog.[1] Animals start from eggs, it seemed.

Furthermore, those eggs seemed to be fertilized by spermatozoa, yet it took a number of people and many observations to observe that a sperm cell actually combines with an egg cell. George Newport wrote three lengthy descriptions of his observations and experiments to discover how the spermatozoa “impregnate” eggs, concluding that they carry some force of “vitalization,” or process of coming alive.[2] [3] It took a few more decades for Oscar Hertwig to report observations of sperm cells actually entering into, combining with, and thereby fertilizing egg cells. He observed in detail each step of the entry process, as well as appearance of two nuclei and then reduction and division into one nucleus for the fertilized egg.11 By the time of Hertwig’s work in the 1870s, it had become clear that fertilization involves the process of two cells coming together to make one cell.

Edmund Beecher’s Atlas of Fertilization and Karyokinesis[4] in 1895 presented the process of fertilization photographically. He showed the details of sea urchin egg cells combining with sperm cells, reduction division of nuclei, chromosomal and cytoplasmic changes in preparation for cell division, and then the process of cleavage itself. One cell divides into two, into four, and so on. Wilson gave his reader photographs, taken in collaboration with the photographer Edward Learning, and sketches of the key details to highlight essential features of the process. By the end of the nineteenth century, it was clear to biologists that an individual organism’s life began as cells, which underwent fertilization and then divided and differentiated into a complex organism. The egg and sperm cell, and the cells resulting from cell division had begun to have a biological life of their own.

  • [1] von Baer (1827).
  • [2] Newport (1851/1853/1854).
  • [3] For discussion of this point, see for example: Churchill (1970).
  • [4] Wilson (1895).
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