PP- ED

Across all species, there are three categories in which a fetus develops in utero. While each are different on their own accord, they can all be traced back to the original embryonic egg called microlecithal development. To be classified as microlecithal the eggs must have very little yolk, divide uniformly (2,4,8,16, etc.), be of similar size, and go through a complete division before the next stage of development can begin. Organisms that practice this mode of development belong to the amphioxi and lampreys. Mesolectihal development arose next in amphibians like frogs and salamanders. This development is characterized by the formation of two poles in an egg, one being an animal pole and the other being a vegetal pole. The animal pole is the sight of active equatorial division whereas the vegetal pole seldomly divides. Amniotes gave rise to the third and final category of development. Macrolectihal development is categorized by a very large yolk sac on which the embryo develops. The top of the egg has a very small disc of rapidly dividing cells which gives rise to the embryo. Once the embryo forms, it envelops the yolk sac and draws nutrients from it so that it can later form surrounding materials and organelles for waste and gas exchange. This process is very similar to that of placental organisms. The only difference is that placental mammals secondarily derived a microlecithal process from the macrolecithal mode of development.