Which stage of development is argued to be the least variable (in terms of what development alterations can occur) and why?

MCQs on the field of developmental biology from its origins to the present day, from seminal historical experiments to modern techniques and the mechanisms of development. Covers a variety of organisms from their start at fertilization through the stages of their development and on to entire organismal and post-embryonic development, as well as the molecular and genetic regulations involved in these processes.