6A. Model of passive metastasis. 6B. SUM 149 cell spheroids expressing RFP. 6A. Tumor cells secrete unknown differentiation factors, stimulating vasculogenesis and resulting in a cluster of tumor cells, termed embolus, located within the de novo formed vessel. The embolus maintains cell-cell attachments as it moves through the vessel and lodges within a dermal lymph node. 6B. 100× confocal projection image (10 images, Z = 20 μm) of SUM 149 cell spheroids stably expressing RFP in 3-dimensional matrigel culture after 5 days.