This chapter provides an overview of the theories of generation and hereditary resemblance found in Aristotle’s work On the Generation of Animals. This treatise completes the project of explaining the development of the perfected living being, which the epigenetic process of embryological development aims for. Aristotle’s explanation of how a new animal comes into being fits to his four-causal scheme, by adding in the more specific principles, male and female. The opponent, thinks Aristotle, is wrong to think that the generative contributions of the parents (seed, sperma) derive from all parts of the body. Instead, what male and female contribute is the most refined nourishment that their bodies produce, which is ready to become all the parts of the body. Male and female roles are then differentiated: the female provides this blood-derived product to serve as that material body (material cause) while the male’s seed is further refined so as to initiate and direct that development as the efficient cause. Aristotle also explains how it is that particular animals end up as male or female and come to resemble their blood relatives. The chapter ends by reflecting on Aristotle’s sexism in his theory of generation.