Harvey and Highmore experimented together on chick fetuses at Oxford in the early 1640s, yet in 1651 published significantly different treatises on generation that emphasize their reliance on observations and dissections of fetal chicks at different stages of incubation. The key differences follow from their views on matter and souls. Harvey conceives of living bodies as governed by Aristotelian souls and faculties. Highmore views matter as made of corpuscles and describes organs as involved in chemical procedures. Highmore's treatise is a response to Digby's claim that heat, moisture, and pressure could explain generation. Although Digby's treatment lacks Harvey's and Highmore's attention to detail, it offers a point of comparison that leads to a more nuanced understanding of their explanation. Moreover, Highmore's dedication of his work to Boyle provides a new perspective on both men's intellectual evolution from the latter part of the 1640s to 1651.