Imagine you are studying neural tissue specification in the Xenopus model. The dorsal (“top”) part of the Xenopus blastula, called the “animal cap”, normally forms ectoderm-derived neural tissue and epidermis.

When the animal cap is cut away from the rest of the blastula and grown by itself in culture, it only forms epidermis. This suggests some sort of secreted factor is required for induction of neural tissue. You add a growth factor protein called Noggin to the cultured animal cap and carry out staining for a marker of neural tissue. This is the result, where positive staining is dark:

What conclusion can be drawn? Choose the CORRECT answer.