The traditional modes of reproduction include oviparity, taken to be the ancestral condition, traditionally where either unfertilised
oocytes or fertilised eggs are spawned, and
viviparity traditionally including any mechanism where young are born live, or where the development of the young is supported by either parent in or on any part of their body. However, the biologist
Thierry Lodé recently divided the traditional category of oviparous reproduction into two modes that are distinguished on the basis of the relationship between the
zygote (fertilised egg) and the parents: •
Ovuliparity, in which
fertilization is
external, is taken to be the ancestral condition as a rule; the eggs that the female releases into the environment contain unfertilised
oocytes, and the male fertilizes them by releasing
sperms in proximity to the eggs. In whichever form they are laid, the eggs of most ovuliparous species contain a substantial quantity of
yolk to support the growth and activity of the embryo after fertilization, and sometimes for some time after hatching as well. In all but special cases of both ovuliparity and oviparity, the overwhelming source of nourishment for the embryo is the
nutrients stored in the yolk, pre-deposited in the egg by the
reproductive system of the mother (the
vitellogenesis). Offspring that depend on yolk in this manner are said to be
lecithotrophic, which literally means "feeding on yolk"; as opposed to
matrotrophy, where the maternal circulation provides for the nutritional needs. Distinguishing between the definitions of oviparity and ovuliparity necessarily reduces the number of species whose modes of reproduction are classified as oviparous, as they no longer include the ovuliparous species such as most fish, most frogs and many invertebrates. Such classifications are largely for convenience and as such can be important in practice, but speaking loosely in contexts in which the distinction is not relevant, it is common to lump both categories together as just "oviparous". ==References==