The blastula (from Greek βλαστός blastos "seed", "Bud", "shoot") is an early embryonic stage of multicellular animals , responsive to the morula follows stage created, and the cleavage terminates. In a narrower sense, only the "bladder germ" that occurs in many animals and that surrounds a fluid-filled cavity, the so-called blastocoel , is referred to as a blastula . After the formation of the blastula, the blastulation , develops as a subsequent stage, a multilayer gastrula , the germinal system of which is often irreversibly divided into two to three cotyledons .
The coeloblastula or archiblastula, also called bladder germ, is often spherical or egg-shaped and characterized by a more or less large centrally or eccentrically located cavity, the blastocoel. It is often viewed as the typical blastula. It can be single-layered ( e.g. in the case of many tissue-less and hydrozoans , as well as moss animals , tardigrade , sea cucumbers and skullless animals ) or multi-layered (e.g. in some scorpions , all sturgeons and amphibians ). In the case of a severely flattened, single-layer coeloblastula, the term placula is often used, for example in the case of some cnidarians and little bristles . In sea urchins , string worms and hedgehog worms , premature formation of the middle cotyledon ( mesoderm ) can occur during blastulation .
The cavity is missing in a sterroblastula. Here, too, there are single and multilayered blastulae. Examples of this are flower animals , acorn worms and syringe worms . Bald pike (genus Amia ) have a multilayered sterroblastula with syncytial nuclei.
A discoblastula ("germ disc") arises in animal species with discoidal furrows , for example in reptiles , birds and monotremes .
In animal species with a superficial furrow one speaks of a periblastula. Since most insects, as well as some other animals, furrow in this way, the periblastula is the most common blastula type in terms of the number of species.
Special case of blastocyst
Marsupial mammals and higher mammals do not form a blastula, but a blastocyst . At this stage, a cluster of cells is already differentiating at one point on the wall of the hollow sphere, from which the embryo later develops ( embryoblast ), while the rest of the germ ( trophoblast ) develops into auxiliary embryonic organs. The blastocyst cavity is not homologous with the blastocoel of the coeloblastula, but corresponds to the early formation of the interior of the yolk sac of reptiles, birds and monotremes. This is why the blastocyst is often contrasted with the blastula.
- P. Fioroni: General and Comparative Embryology of Animals . Springer, Berlin 1992, ISBN 3-540-55495-5 .