A veterinarian presents with a chief complaint of thin, watery, non-bloody diarrhea for the past two weeks. He appears fatigued and mildly dehydrated. Stool examination is negative for leukocytes and culture for bacterial pathogens is negative. Acid fast stain of the stool demonstrates oocysts that are 5-7 microns in diameter. Where does the sexual phase of this organism's life cycle occur?