The first stage in the development of an embryo,formed by cleavage of the zygote. It consists of a hollow sphere containing undifferentiated dividing cells (blastomeres), which form a layer (blastoderm) around a central cavity (blastocoele). In mammals, the embryo at this stage is referred to as the blastocyst (see figure below).
Blastula showing the blastomeres (inner cell mass) and blastocoele. Trophoblast is endothelium surrounding the blastula, forming and outer layer of chorion, and which becomes part of the embryonic component of the placenta, or of the extra-embryonic membranes
See Adhesion molecules, Archenteron, Blastomere, Chorion, Cleavage, Ectoderm, Embryogenesis, Entoderm (or endoderm), Epithelium, Gastrulation, Methylation, Morula, Placenta, Zygote