By the formation of an inner cavity or blastocoele, the blastocyst or blastula comes into being.    From Wordnik.com.  [Hans Spemann - Nobel Lecture] Reference

It becomes even more obvious when a piece of the roof of the archenteron is planted in the blastocoele.    From Wordnik.com.  [Hans Spemann - Nobel Lecture] Reference

The gastrulation does not suffer any essential disturbance from this and while it goes on, the blastocoele disappears and the piece we are examining comes to lie directly under the ectoderm and there shows what it is capable of.    From Wordnik.com.  [Hans Spemann - Nobel Lecture] Reference

That is to say, those parts of the embryo which are being examined for their inductive capacity can be made to bypass the activte invagination and can be made effective by inserting them in the blastocoele through a small slit in the roof of the blastula or young gastrula which quickly heals over.    From Wordnik.com.  [Hans Spemann - Nobel Lecture] Reference

Thus a portion of the upper marginal zone of the blastula or early gastrula, or else a piece of the roof of the archenteron of the mature gastrula was planted in the blastocoele of a young gastrula and so brought beneath the ectoderm from the beginning; it was demonstrated that these portions were able to induce neural plate.    From Wordnik.com.  [Hans Spemann - Nobel Lecture] Reference