Anatomy is also offered in the fall semester. Generally, osteopathic students take it in the summer and allopathic students take it in the fall. It MAY be possible to take it in the fall, but they will probably discourage it (I mean they will tell you NO). Having two med schools limits the space plus MSUCOM believes that having anatomy separate from your other classes lets you focus on anatomy first and also allows you to take less classes in the fall (who wants 20 some credits?).
If you absolutely need to start in the fall, I would contact them.