to classify it as a protostome, zoologist must see for the mouth development in the embryo.
Explanation:
The protostome are the animals which are unique to their three germ layer in embryonic development, with the bilateral symmetry as they have the ability to fly with other protostome developments.
the one characteristic feature to distinguish them as the protostome, mouth part development is to be sought as the mouth develops but not the anus in the embryo. the mouth parts serves unique to the group and their feeding structures.
The directions/steps given might be too vague, the scientist made an error, or the experiment was badly written
Bivariate cell cycle assay (BrdU/PI) using cell cycle-specific antibodies and DNA dyes can distinguish G0 from G1 phase or G2 from M phase
https://www.creative-bioarray.com/bivariate-cell-cycle-assay-brdu-pi.htm