The Collaborative International Dictionary of English v.0.48:

Blastopore \Blas"to*pore\, n. [Gr. blasto`s sprout + E. pore.]
   (Biol.)
   The pore or opening leading into the cavity of invagination,
   or archenteron.

   Note: [See Illust. of Invagination.] --Balfour.
         [1913 Webster]

WordNet (r) 3.0 (2006):

blastopore
    n 1: the opening into the archenteron