Early embryos of many species develop ancestral features.

很多物种的早期胚胎会发展祖先的特征。

If for any reason this does not happen, the ancestral feature may not disappear, leading to an atavism.

如果因为某个原因这种现象没有发生，这种祖先的特征就可能不会消失，产生返祖。