Human Reproduction

Early in development, the embryo must establish its body axes: anterior–posterior (head–tail), dorsal–ventral (back–belly) and medial–lateral (left–right).
Spemann and Mangold showed in amphibian embryos that a special region, the dorsal lip of the blastopore (the organizer), induces and organises the formation of the main body axis and the neural tube. When it was grafted into another embryo, a second body axis formed.
This inducing and organising action of the organizer, which sets up the polarity of the embryo, is called the organizer phenomenon.
As per the answer key, the establishment of polarity along these axes is called the organizer phenomena. (In modern textbooks the term axis formation is also used for this process.)