Animal models are crucial to further our understanding of tumor biology. For many years, observation chambers, implanted in various animal species, have been used for intravital microscopy of tumor microcirculation. As far back as 1943, Algire described the adaptation of the transparent chamber technique to the mouse (1 ). From the introduction of a subsequent paper by Algire et al. (2 ), the following paragraph is quoted: “Recently, in vivo techniques have been developed which make it possible to make microscopic observations of the development of the blood supply of tumor transplants in the mouse. It has also been possible to obtain a quantitative expression of the morphological changes as they occur over a prolonged period? The results presented indicate (i) that the rapid growth of tumor transplants is dependent upon the development of a rich vascular supply, and (ii) that an outstanding characteristics of the tumor cell is its capacity to elicit continuously the growth of new capillaries.”