Some definitions from Bartleby.com reference dictionary
NOUN: Inflected forms: pl. blas·tu·las or blas·tu·lae (-l)
An early embryonic form produced by cleavage of a fertilized ovum and consisting of a spherical layer of cells surrounding a fluid-filled cavity. Also called blastosphere.
ETYMOLOGY: New Latin : from Greek blastos, bud + Latin -ula, feminine diminutive suffix
It is a hollow ball of cells, one cell thick. (Dawkins, below)
NOUN: Embryology The opening of the archenteron.
NOUN: Inflected forms: pl. gas·tru·las or gas·tru·lae (-l)
An embryo at the stage following the blastula, consisting of a hollow, two-layered sac of ectoderm and endoderm surrounding an archenteron that communicates with the exterior through the blastopore.
ETYMOLOGY: New Latin : Greek gastr, gastr-, belly + Latin -ula, feminine diminutive suff
NOUN: The process of forming a gastrula.
NOUN: The central cavity of the gastrula, which ultimately becomes the intestinal or digestive cavity.
In protostomes (mouth first) the blastopore becomes the mouth (or both mouth and anus. In deuterostomes (mouth second) the blastopore becomes the anus.
See Dawkins, AT, pp.387-89