If in an eight-cell embryo of an organism, the cells of the upper tier are aligned over the cells of the lower tier at a slight angle, it is likely that the organism is a vertebrate.
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Generally, epithelial cells serve the function of protection of the inner layers of the structure they are on. The fact that these columbar epithelium and transitional epithelium cells have smooth walls and are dome shaped allows them to stretch. This stretching helps the bladder retain more urine than it would be able to if the epithelial cells were rigid.
