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Fertilization is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside in the case of external fertilization.
A relatively short time after fertilization, the cortical cytoplasm, located just beneath the cell membrane, rotates by 30 degrees, which results in the creation of the gray crescent; its establishment determines the location of the dorsal and ventral (up-down) axis, as well as of the anterior and posterior (front-back) axis and the dextro ...
The term ichthyopsida means fish-face or fish-like as opposed to the Sauropsida or lizard-face animals (reptiles and birds) and the mammals. [6] The group representing an evolutionary grade rather than a clade , the term anamniote is now used as an informal way of denoting the physical property of the group, rather than as a systematic unit.
Whatever the reason, the drive led to internal fertilization and direct development (completing the tadpole stage within the egg). A striking parallel can be seen in the frog family Leptodactylidae, which has a very diverse reproductive system, including foam nests, non-feeding terrestrial tadpoles and direct development. The Diadectomorphans ...
In amphibians, the development of the animal-vegetal axis occurs prior to fertilization. [1] Sperm entry can occur anywhere in the animal hemisphere. [2] The point of sperm entry defines the dorso-ventral axis - cells opposite the region of sperm entry will eventually form the dorsal portion of the body. [1] [3]
The young of ovoviviparous amphibians are sometimes born as larvae, and undergo metamorphosis outside the body of the mother. Modes of reproduction include [3] based on relations between zygote and parents: Ovuliparity: external fertilisation, as in arthropods, many bony fishes, and most amphibians
The ZW sex-determination system is a chromosomal system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects (including butterflies and moths), the schistosome family of flatworms, and some reptiles, e.g. majority of snakes, lacertid lizards and monitors, including Komodo dragons.
After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst the amniote ancestors, the next major breakthrough appears to have involved a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both ...