Isolecithal (Greek iso = equal, lekithos = yolk) refers to the even distribution of yolk in the cytoplasm of ova of mammals and other vertebrates, notably fishes of the families Petromyzontidae, Amiidae, and Lepisosteidae.
However, during cellular development, normally under the influence of gravity, some of the yolk settles to the bottom of the egg, producing an uneven distribution of yolky hemispheres.
Such uneven cells are known as telolecithal and are common where there is sufficient yolk mass.
These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis.
In mammals, because the isolecithal cells have only a small amount of yolk, they require immediate implantation onto the uterine wall to receive nutrients.