Cleavage (embryo)

As segmentation is called cell division by pinch in zygotes (fertilized oocytes) at the beginning of embryogenesis ( embryonic ) of multicellular animals. Here, the embryo does not increase. So there is no formation of new material is needed, it is in these divisions are the fastest ever, a nuclear division to occur every eight minutes. After a large number of corrugations to form as a densely filled with cells ball, the morula. These drawings corrugations cells called blastomeres. One difference to the "normal " cell divisions is also that all divisions are synchronous. Furthermore, the nuclear - cytoplasmic ratio is constantly changing as it otherwise remains constant during cell division. The cell cycle is shortened, it consists only of S- and M- phase, resulting in the enormous speed of cell divisions can be explained.

Furchungstypen

There are three Furchungstypen: Holoblastische, Meroblastische and Wendelfurchung.

Holoblastische furrowing

(Greek holos, whole ') - complete cleavage

• Isolecithal - yolk uniformly distributed (Greek iso, uniformly ') radial: echinoderms, the lancelet
• Bilateral: tunicates
• Rotationally: mammals, nematodes

• Radially offset: amphibians

Meroblastische furrowing

(Greek meros, part ') - partial cleavage

• Telolecithal - yolk concentrated at one pole (Greek telo 'end' ) discoidal: cephalopods, fish, reptiles ( and monotremes ), birds

• Superficial: Insects

Wendelfurchung

• Isolecithal Helical: The cells are staggered helical.

When Wendelfurchung that (except cephalopods ) is for annelids and molluscs characteristic that Furchungsspindeln be arranged at an oblique angle to the animal - vegetative axis, so that the blastomeres are helically offset from each other.