Question

consisting of 3 layers of cells, the gastrula - stage embryo begins as ________. a flat sheet a hollow tube a folded sheet a hollow sphere

Explanation:

During embryonic development, the blastula (a hollow sphere of cells) undergoes gastrulation to form the gastrula. The gastrula - stage embryo begins as a hollow sphere (the blastula) which then transforms during gastrulation. A flat sheet is not the starting structure for gastrulation. A hollow tube is not the initial structure. A folded sheet is a result of some morphogenetic movements during development but not the starting point. The blastula (a hollow sphere) is the immediate precursor to the gastrula.

Answer:

a hollow sphere