The Wolffian, or male, duct structures do not form because of the lack of response to androgens. The Müllerian, or female, duct structures do not evolve because the testes still release MIS.

These are the primitive sex cells that will form eggs and sperm. If bird becomes male, Wolffian ducts develop into vas deferens or sperm duct and Mullerian ducts atrophy. The reverse occurs in the ...