From WordNet (r) 2.1 (2005) (link)

  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]

From Merriam-Webster's Collegiate Dictionary, 11th Edition (2003) (link)

  blastula noun (plural -las or blastulae)
   Etymology: New Latin, from Greek blastos bud — more at
   -blast Date: 1879
    an early metazoan embryo typically having the form of a hollow fluid-filled
   rounded cavity bounded by a single layer of cells — compare gastrula,
   morula • blastulation noun
  
  

From Oxford English Reference Dictionary (link)

blastula
n. (pl. blastulae or US blastulas) Biol. an animal embryo at an early stage of development when it is a hollow ball of cells.
Etymology: mod.L f. Gk blastos sprout

From English Explanatory Dictionary (link)

  blastula
  n. (pl. blastulae or US blastulas) Biol. an animal embryo at an early stage
  of development when it is a hollow ball of cells. [mod.L f. Gk blastos sprout]
  
  
  

From Webster's Revised Unabridged Dictionary (1913) (link)

  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.