part of part of developmental process involved in reproduction multicellular organism development sex differentiation development of secondary sexual characteristics biological_process GO:0045136 The process whose specific outcome is the progression of the secondary sexual characteristics over time, from their formation to the mature structures. In humans, these include growth of axillary, chest, and pubic hair, voice changes, testicular/penile enlargement, breast development and menstrual periods. Development occurs in response to sex hormone secretion.