Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
Related to Blastula: gastrulation, archenteron


A hollow sphere of cells characteristic of the early metazoan embryo.



a stage in the development of the embryo of multicellular animals culminating in cleavage.

The structure of the embryo in the blastula stage depends on the structure of the egg and type of cleavage. Radial cleavage (echinoderms, lancelets, amphibians, sturgeon) gives rise to a coeloblastula, a spherical embryo with a cavity, or blastocoel, which is filled with a fluid differing in chemical composition from the medium surrounding the embryo. The blastocoel is sometimes in the center of the blastula but is generally shifted to the animal (upper) part of the embryo. The blastular wall, the blastoderm, consists of one, several, or many rows of cells. In animals with the spiral type of cleavage (most mollusks and some worms, for example), the blastula is formed without a cavity, a sterroblastula. Animals with incomplete (partial) discoidal cleavage (bony fishes, sharks, reptiles, birds) form a discoblastula whose cavity is reduced; the upper wall consists of many rows of cells while the lower uncleaved wall contains the yolk. Partial superficial cleavage (some arthropods) gives rise to a blastula in which the cavity is filled with yolk.

Cleavage of eggs in mammals and man ends in the formation of blastocysts rather than blastulas.

References in periodicals archive ?
Only cleavage and early blastula stage embryos were retained for further study.
On the other hand the fact that the resistance to environmental agents could change significantly even within each developmental stage as it was reported also for the amphibian blastula, (31) seems to justify to revisit the criteria on which the developmental process is divided in stages by taking into account the changes in the susceptibility to physico chemical agents as biomarkers for that purpose.
Karyotype analysis of blastula stage larvae from the 3n[female] x 2n[male] cross revealed a high level of aneuploidy, with chromosome number ranging from 11-87.
Developmental Stage Time(h or days) Oviposition 0 h First division 6 [+ or -] 1 h of cleavage Four-cell stage 8 h Eight-cell stage 11 h Morula 16 h Blastula, appearance of blastocoel 2 days Gastrula 2 days Motion of larvae by velar rudiment 3 days Early veliger stage, velum visible, viscera 4 days differentiated in stomach, left and right digestive diverticulum, shell color changed to purple Hatched 6 days No.
Next comes gastrulation, the crucial step in which the blastula rearranges itself into three populations of cells: a surface layer, the ectoderm; a middle layer, the mesoderm; and an inner layer, the endoderm.
These stages proceed until a formation of multicellular blastula is achieved.
Twenty-four hours after mating, embryos at the early-cleavage, blastula, gastrula, neurula, and tadpole stages were collected, preserved in Bouin's fixative, sputter coated with a mixture of platinum and gold, and observed using a scanning electron microscope (S-2460N, Hitachi).
The blastula stage occurs 3-14 days after fertilization.
On day 0, fathead minnow embryos (n = 240) at blastula and morula developmental stages were gathered from mating tiles in the husbandry unit at Brixham Environmental Laboratory.