“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