The Reproductive Process
“Omne vivum ex ovo”
In 1651, late in a long life, William Harvey, the English physiologist who earlier had ushered in experimental physiology by explaining the circuit of the blood, published a treatise on reproduction. He asserted that all life developed from the egg—omne vivum ex ovo. This was curiously insightful, since Harvey had no means for visualizing the eggs of many animals, in particular the microscopic mammalian egg, which is no larger than a speck of dust to the unaided eye. Further, argued Harvey, the egg is launched into its developmental course by some influence from the semen, a conclusion that was either remarkably perceptive or a lucky guess, since sperm also were invisible to Harvey. Such ideas differed sharply from existing notions of biogenesis, which saw life springing from many sources of which eggs were but one. Harvey was describing characteristics of sexual reproduction in which two parents, male and female, must come together physically to ensure fusion of gametes from each.
Despite the importance of Harvey’s aphorism that all life arises from eggs, it was too sweeping to be wholly correct. Life springs from the reproduction of preexisting life, and reproduction may not be restricted to eggs and sperm. Nonsexual reproduction, the creation of new, genetically identical individuals by budding or fragmentation or fission from a single parent, is common, indeed characteristic, among some phyla. Most animals have found sex to be the winning strategy, probably because sexual reproduction promotes diversity, enhancing long-term survival of the lineage in a world of perpetual change.
Reproduction is one of the ubiquitous miracles of life. Evolution is inextricably linked to reproduction, because the ceaseless replacement of aging predecessors with new life gives animals the means to respond and evolve in a changing environment as the earth itself has changed over the ages. In this section we distinguish asexual and sexual reproduction and explore the reasons why, for multicellular animals at least, sexual reproduction appears to offer important advantages over asexual. We then consider, in turn, the origin and maturation of germ cells; the plan of reproductive systems; the reproductive patterns in animals; and, finally, the endocrine events that orchestrate reproduction.
In 1651, late in a long life, William Harvey, the English physiologist who earlier had ushered in experimental physiology by explaining the circuit of the blood, published a treatise on reproduction. He asserted that all life developed from the egg—omne vivum ex ovo. This was curiously insightful, since Harvey had no means for visualizing the eggs of many animals, in particular the microscopic mammalian egg, which is no larger than a speck of dust to the unaided eye. Further, argued Harvey, the egg is launched into its developmental course by some influence from the semen, a conclusion that was either remarkably perceptive or a lucky guess, since sperm also were invisible to Harvey. Such ideas differed sharply from existing notions of biogenesis, which saw life springing from many sources of which eggs were but one. Harvey was describing characteristics of sexual reproduction in which two parents, male and female, must come together physically to ensure fusion of gametes from each.
Despite the importance of Harvey’s aphorism that all life arises from eggs, it was too sweeping to be wholly correct. Life springs from the reproduction of preexisting life, and reproduction may not be restricted to eggs and sperm. Nonsexual reproduction, the creation of new, genetically identical individuals by budding or fragmentation or fission from a single parent, is common, indeed characteristic, among some phyla. Most animals have found sex to be the winning strategy, probably because sexual reproduction promotes diversity, enhancing long-term survival of the lineage in a world of perpetual change.
Reproduction is one of the ubiquitous miracles of life. Evolution is inextricably linked to reproduction, because the ceaseless replacement of aging predecessors with new life gives animals the means to respond and evolve in a changing environment as the earth itself has changed over the ages. In this section we distinguish asexual and sexual reproduction and explore the reasons why, for multicellular animals at least, sexual reproduction appears to offer important advantages over asexual. We then consider, in turn, the origin and maturation of germ cells; the plan of reproductive systems; the reproductive patterns in animals; and, finally, the endocrine events that orchestrate reproduction.
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