How did William Harvey compare reproduction in chickens and deer?

In his studies of animal reproduction, William Harvey utilized chickens and deer to compare different developmental processes. He examined chickens as a primary example of oviparous reproduction, where the embryo develops within an egg that is hatched outside of the mother's body. In contrast, he studied deer as an example of viviparous reproduction, a process in which the embryo develops internally, leading to the birth of live young.1

Through these comparative observations, which were published in his 1651 work Exercitationes de Generatione Animalium, Harvey sought to understand the formation of the embryo, including the roles of sperm and menstrual blood. While his observational detail was significant, Harvey was limited by the technology of his time, as these complex biological questions could not be fully resolved without the use of a microscope.21