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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 ?
SC females produced blastulae with more fat than did females from FL-SM on the same food treatment.
9%) with moderate sample sizes (18 broods at the blastulae stage and 11 at the neonate stage).
Wrinkled blastulae have been observed in a number of echinoderms with nonfeeding larvae.
Though we did not notice any instances in which two embryos developed within the same fertilization envelope, our observations of relatively large distances between the cells of developing embryos, our inability to see a hyaline layer, and our finding that embryos that were irregularly shaped after fourth cleavage later formed regular blastulae certainly concur with those of Mortensen (1938) and Schroeder (1981).
A more detailed analysis of the different polysome size classes was performed for blastulae (Fig.
Until now, it has been thought that wrinkled blastulae formed only in direct-developing sea urchins and not in indirect-developing species (1).
We usually fed Isochrysis galbana to our cultures after blastulae hatched, but we could not see autofluoresence of the algae in the gut, and cultures developed to similar stages even when they were not fed.
In a laboratory study, I have quantified sinking and swimming speeds of early blastulae from four echinoid species whose similar early development makes them a natural experimental system for investigating swimming behavior.
Moreover, models of vertical swimming by flagellates in a wind-mixed water column (Yamazaki and Kamykowski, 1991) indicate that even the slow upward swimming of sea urchin blastulae (Mogami et al.
Blastulae were obtained on 19 February 1997 from a single egg mass maintained in an aquarium; about 70 embryos were in this tubular mass.