The precise way in which form relates to matter, however, is complex and resists easy summation. We might rather point to a few contexts in which Galen invokes form in physical explanations. In embryology, form represents the component of design (in modern terms, of information) which is transmitted to the embryo from the parents, as opposed to the matter out of which it is constructed. It thus becomes an explicit question (problematized in different ways both in the early text Semen (Sem.) and in the late one Foet. Form.), how the form is present in the matter, and what is the nature of the process of interaction. In the former text, for example, the process crucially involves the interaction of pneuma (breath, containing the formal element) and blood (containing the appropriate matter); and a distinction is made between ousia (in the material sense) and the power that moves and shapes it.