The yolk plug of a gastrula represents which part in the later stage of development?

If the ectoderm from the neural tissue area is removed from the embryo and transplanted in place of the presumptive belly ectoderm,the embryo will develop:

The cleavage in a frog is:

The grey crescent is the area:

Which one of the following parts serves the function of primary organizer in the frog embryo?

The middle gastrula stage is characterized by: