blastula /blas″tyələ/ [Gk, blastos, germ] , an early stage of the process through which a zygote develops into an embryo, characterized by a fluid-filled sphere formed by a single layer of cells. The spheric layer of cells is called a blastoderm; the fluid-filled cavity is the blastocoele. The blastula develops from the morula stage and is usually the form in which the embryo becomes implanted in the wall of the uterus. Also called blastosphere.
