This image is a dorsal (back) view of a mouse embryo spinal cord and rib cage prepared by Alcian Blue and Alizarin Red staining for cartilage and bone, respectively. Here, bony regions are turquoise in color while cartilage is light pink. Snakes have up to 400 pairs of ribs extending from all vertebrae, while mice have 13 rib pairs, and humans have 12 pairs. Males and females have the same number of ribs, but in males undergoing puberty, the burst of testosterone causes the rib cage to expand (which allows them to breathe in more air with a single breath, providing more oxygen to their muscles). This image was prepared at the 2011 Woods Hole Embryology Course.