Sentence:  ...phenomenon that is here called sex <WSD>heteronymy</WSD>, which implies that the two sexes of animals (or human beings, for instance relatives) are referred to by words which are altogether dissimilar as in bull: cow, {aunt: uncle) Deer. 
Meaning of the word marked with <WSD> tags:  The unrelatedness of words for items that are related by being members of a single category.