A 73-year-old man with a history of chronic alcoholism has had increasing difficulty swallowing and has noticed a 3-kg weight loss over the past 2 months. On physical examina- tion, there are no remarkable findings. Upper gastrointestinal endoscopy shows a 3-cm ulcerative mass in the midesophagus that partially occludes the esophageal lumen Esophagectomy is performed; the gross appearance of the lesion is shown in the figure. Which of the following is most likely to be seen on microscopic section of this mass?