There are two different notions of seed (sperma) at work in the Generation of Animals: seed as the spermatic residue (perittôma), which concerns only the male and the female generative contributions, and seed as the kuêma and first mixture of the two generative contributions. The latter is a notion of seed common to plants and animals. The passage in GA I.18, 724b12–22 where Aristotle distinguishes between these two notions of seed has been mistakenly discredited as inauthentic or simply as irrelevant for understanding the seeds of animals. On the other hand, recent studies have rather focused on the seed as spermatic residue (in particular, the female contribution as seed), paying little attention to the notion of seed as kuêma. In this paper I defend the authenticity and relevance of this passage and show how understanding the notion of seed as kuêma is essential to have a complete picture of Aristotle’s account of seed. This common notion of seed makes sense of Aristotle’s otherwise puzzling use of the word “sperma” to designate the seeds of plants, the kuêma, the fertilized eggs, and the first mixture of the two generative contributions. It also proves helpful in determining what the word ‘sperma’ stands for in key passages, such as Metaph. IX.7.