A 44-year-old man comes to the physician because he is worried about a mole on his right forearm. He has had the mole for several years, but it has grown in size in the past 3 months. Physical examination shows a hyperpigmented plaque with irregular borders. Histopathologic analysis of the mole confirms the diagnosis of malignant melanoma. Invasion of which of the following skin layers is most dangerous for the patient in view of the further possible spread of tumor cells?