Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 97

Figure 2.25 Budding in Hydra .
Source : From http://biology.about.com/library/weekly/aa090700a.htm.
release, and union of egg and sperm cells. As already mentioned, metazoans do not
produce copies of themselves, and apparently, this is impossible from the develop-
mental point of view. They produce reproductively specialized cells and egg and
sperm cells. With the exception of the hermaphroditic organism, these cells are
produced from both male and female individuals; hence, the name dioecious (from
Greek δύο ( dyo ), two, and οίκος ( oikos ), house, from proto-Indo-European * wei ΄ ,
to settle) for the species with separate sexes and their mode of reproduction via the
union of egg and sperm cells. The great majority of animal species are dioecious and
reproduce sexually, i.e., via the union of the gametes of opposite sexes, the egg cell
and sperm cell, produced by the female and the male parent, respectively.
A smaller number of metazoan species develop from eggs that per se are capa-
ble of developing into organisms of the kind that produces them. These animal spe-
cies are known as parthenogenetic (from Greek, παρθένος (parthenos), virgin, and
γένεσις (genesis), origin, creation] and this mode of reproduction is called parthe-
nogenesis . Curiously, there are some salamander species that, in adaptive responses
to environmental circumstances, may switch between the dioecious and parthenoge-
netic modes of reproduction.
The principal mechanism of sexual reproduction in metazoans was discov-
ered by the end of the nineteenth century, when biologists in several countries