A 35-year-old woman who underwent a modified radical mastectomy of her right breast for infiltrating ductal carcinoma 2 years ago presents with enlargement of her right breast. The breast has a swollen, red-discolored appearance. It is diffusely indurated and tender on palpation. Multiple axillary lymph nodes are palpable in the lower axilla. The working clinical diagnosis is inflammatory carcinoma. Microscopic sections from this red, indurated area are most likely to reveal