Sentence:  The owl's process of assimilation makes it impossible any longer for the mouse's substantial form to animate the mouse's <WSD>protomatter</WSD>, so it is replaced in its functioning by the substantial form of the owl. A mouse can become an owl (or part of an owl) preciesely because there is inherent in the mouse a principle of determinability that includes a capability to be owl. That principle, <WSD>protomatter</WSD>, is actualized (by the mouse-form) in the mouse, but is potentiality to be actualized in other ways. 
Meaning of the word marked with <WSD> tags:  The ultimate basis of physical substance, before it has been given form.