St. Thomas a Preformationist

From De Principiis Naturæ you can clearly tell that—contrary to many claims that St. Thomas agreed with Aristotle that a human fetus temporally first has a vegetative, sensitive, then intellectual soul—St. Thomas was a "preformationist" as opposed to an "epigeneticist" (cf. this), viz., he argues that man is a substance, a substantial whole, more than just a sum of his parts, more than a collection of accidental forms:

4. [...] matter differs from subject because the subject is that which does not have existence by reason of something which comes to it, rather it has complete existence of itself (per se); just as man does not have existence through whiteness [or through any other accidental forms that comprise man, e.g., his bones, brain, etc.].

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6. [...] Generation simpliciter corresponds to the substantial form [that man is generated simpliciter corresponds to preformationism] and generation secundum quid [This is how epigeneticists think man is generated.] corresponds to the accidental form. When a substantial form is introduced we say that something comes into being simpliciter, for example we say that man comes into being or man is generated [something]. But when an accidental form is introduced, we do not say that something comes into being simpliciter, but that it comes into being as this; for example when man comes into being as white, we do not say simpliciter that man comes into being or is generated, but that he comes into being or is generated as white [somehow].