The Collaborative International Dictionary of English v.0.48:

Blastula \Blas"tu*la\, n. [NL., dim. of Gr. blasto`s a sprout.]
   (Biol.)
   That stage in the development of the ovum in which the outer
   cells of the morula become more defined and form the
   blastoderm.
   [1913 Webster]

WordNet (r) 3.0 (2006):

blastula
    n 1: early stage of an embryo produced by cleavage of an ovum; a
         liquid-filled sphere whose wall is composed of a single
         layer of cells; during this stage (about eight days after
         fertilization) implantation in the wall of the uterus
         occurs [syn: blastula, blastosphere]