Despite this observational effort, Digby's experience with the embryo is quite limited, and his theory of development relates more to his philosophical stance than to the facts of development. Indeed, the theory he propounds is not necessarily consistent. On the one hand, it posits a strictly mechanistic epigenesis, and on the other hand, it incorporates the notion of "specificall vertues drawne by the bloud in its iterated courses, by its circular motion, through all the severall partes of the parents body."[6] Digby rejects an internal agent, entelechy, or the Aristotelian formal and efficient causes. Similarly, he disposes of the idea that the embryonic parts derive from some part of each part of the parent's body or an assemblage of parts. This possibility is eliminated, he contends, by the occurrence of spontaneous generation. If a collection of parts was necessary, he asks, "how could vermine breed out of living bodies, or out of corruption?... How could froggs be ingendered in the ayre?"[7] Generation in plants and animals must, then, according to Digby, proceed from the action of an external agent, effecting the proper mingling of the rare and dense bodies with one another, upon a homogeneous substance and converting it into an increasingly heterogeneous substance. "Generation," he says, is not made by aggregation of like partes to presupposed like ones: nor by a specificall worker within; but by the compounding of a seminary matter, with the juice which accreweth to it from without, and with the streames of circumstant bodies; which by an ordinary course of nature, are regularly imbibed in it by degrees; and which att every degree do change it into a different thing.[8] Digby argues that the animal is made of the juices that later nourish it, that the embryo is generated from superfluous nourishment coming from all parts of the parent body and containing "after some sort, the perfection of the whole living creature."[9] Then, through digestion and other degrees of heat and moisture, the superfluous nourishment becomes an homogeneous body, which is then changed by successive transformations into an animal. Digby is frankly deterministic in his description of embryonic development: