 The first 13 chapters characterize the reproductive organs in the males and females of blooded animals, while 14–16 do the same for the bloodless animals that have males and females. Beginning in chapter 17, the discussion turns to the relevant uniform parts related to generation, sperma (roughly ‘seed’) and milk, but the discussion of milk is postponed until book IV. This immediately raises the contentious issue of whether both male and female contribute sperma, and if both, then what the nature of their respective contributions is. This is a large part of the discussion from I 17-II 5, one of the most difficult stretches of text in the Aristotelian corpus, and the one on which our attention will be focused. But to continue our overview: the remainder of book II (from chapter 5 on) discusses the causes of the embryological development of live-bearing animals, while the egg-laying animals are the primary focus in book III, which closes with a discussion of animals that are not sexually generated, including those that arise ‘spontaneously.’ GA IV turns to the development of inherited differences within kinds, beginning with the most important intra-specific difference for his theory of generation, the difference between male and female. From there it moves on to family resemblance, differences in the number of offspring produced and to the causes of deformities. GA V aims to explain ‘more and less’ variations within kinds that appear late in development and that may be fully accounted for by identifying material level causal interactions—differences within a kind in the color, texture or density of a part, for example.