Oviparous Mammals Special Viviparous
Oviparity, egg-laying, is considered to be the ancestral mode of reproduction for vertebrates. Ova (eggs) develop in the females’ ovaries (in rare exceptions in males, such as a clade of toads, and pathologically in other vertebrates), mediated by such hormones as estrogen and progesterone, luteinizing hormone, etc., usually cyclically and often determined by seasonality (light, moisture, etc.).
Egg sizes vary enormously, often within classes, families, and genera, but size usually characterizes each species. A major feature of egg development is vitellogenesis, the deposition of yolk in each ovum. The ancestral condition is characterized by the development of yolky eggs that are laid externally (in water or on land), often in large numbers, and fertilized externally during a diversity of forms of interactions with males.
The yolk mass present in each ovum is typically sufficient to maintain the developing embryo until it hatches, when it becomes a free-living, usually aquatic, larva that can forage on its own until it metamorphoses. However, in direct-developers (see below) eggs sizes are usually large, so that the embryo is provisioned with enough yolk to carry it through metamorphosis to hatch as a juvenile. Conversely, many viviparous species (see below) have very small eggs with little yolk, correlated with their providing other forms of nutrition to the developing young (e.g., via a placenta, etc.). Numbers of eggs (clutch size) laid vary enormously; some fishes and frogs lay hundreds, even thousands, of eggs that are fertilized in water by one, sometimes more, males.
While egg-laying is the ancestral condition retained by some members of all classes of vertebrates, (most fishes, most amphibians, most squamates, all turtles and crocodiles, all birds, and both species of monotremes among mammals), some are live-bearers (see below).
Also, internal fertilization via a diversity of means of sperm transfer has evolved in most lineages – some fishes, using modified fins, a very few frogs, most salamanders, and all caecilians among amphibians, and all of the amniotes – those vertebrates (reptiles and birds, mammals) that surround their developing embryos with extra-embryonic membranes and then either lay the eggs/embryos or retain them in their oviducts for part or all of their development.