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The formation of a segmentation cavity or blastocoele within a mass of cleaving blastomeres and rearrangement of blastomeres around this cavity in such a way as to form the type of definitive blastula characteristic of each species. The blastocoele originates as an intercellular space which sometimes arises as early as the four- or eight-cell stage. Thus blastulation is initiated during early cleavage stages, and formation of the definitive blastula is thought to terminate cleavage and to initiate gastrulation. Initially the diameter of the blastula is no greater than that of the activated egg; subsequently it increases. See Gastrulation

The blastula is usually a hollow sphere. Its wall may vary from one to several cells in thickness. In eggs which contain considerable amounts of yolk the blastocoele may be eccentric in position, that is, shifted toward the animal pole. The animal portion of its wall is always completely divided into relatively small cells, whereas the vegetative portion tends to be composed of relatively large cells and may be incompletely cellulated in certain species. The blastocoele contains a gelatinous or jellylike fluid, which originates in part as a secretion by the blastomeres and in part by passage of water through the blastomeres or intercellular material, or both, into the blastocoele.

The wall of the blastula is a mosaic of cellular areas, each of which will normally produce a certain structure during subsequent development. In other words, each area of cells in the wall of the blastula has a certain prospective fate which will be realized in normal development.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the concluding phase of cleavage of the mammalian egg during which the blastula is formed.

During blastulation the surface cells of the embryo acquire the appearance of an epithelial layer, the central cavity (blastocoel) appears, cleavage of the blastomeres becomes asynchronous, the mitotic cycle lengthens, and the mitotic index decreases while the duration of the interphase increases. During blastulation nucleoli appear in the interphase nuclei of the cells and the synthesis of information ribonucleic acid (IRNA) starts, triggering the transition to gastrulation.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


Formation of a blastula from a solid ball of cleaving cells.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
In mice, the long-term dietary supplementation of melatonin, between 10 and 43 weeks of age, was also able to mitigate the phenotype of the ageing oocyte with the restoration of oocyte numbers, fertilisation rate, and blastulation rate.
In our study, the time of blastulation was accelerated in FBS supplemented groups compared with those supplemented by BSA during IVM (data not shown).
Differential patterns of blastulation in bovine morulae cultured in synthetic oviduct fluid medium containing FCS or BSA.
However, there was a statistically significant increase (p < 0.05) for all embryo developmental parameters (early dividing embryos, good quality (day 2 and 3) and blastulation rate) in group B.
(29) In the prospective randomised study we found a higher, although not significant, blastulation and pregnancy rate in group B.
There was no statistically variation (p>0.05) in the blastulation rate between control (35.1%), 15 (30.5%), 31 (27.1%), 62 (33.1%) ppm in GLA and 250 ppm LAA (32.4%).
In the present study we found that the optimum concentration of GLA was 62 ppm, based on the blastulation rate.