The CAM is essentially a 2D lung that sits atop of the embryo.  Below is is the embryo, yolk sac, and alot of albumin.  This is the site of gas exchange and as is the case for alot of cancer patients, this lung is a great place to implant tumors.  Because it is so highly vascularized, we can inject fluorescent cancer cells and then image their migration throughout the microvasculature.  The best model for intravital imaging of cancer cells in my opinion.