Full context:   All vertebrate brains share a common underlying form, which appears most clearly during early stages of embryonic development. In its earliest form, the brain appears as three vesicular swellings at the front end of the neural tube; these swellings eventually become the forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon), respectively. At the earliest stages of brain development, the three areas are roughly equal in size. In many aquatic/semiaquatic vertebrates such as fish and amphibians, the three parts remain similar in size in adults, but in terrestrial tetrapods such as mammals, the forebrain becomes much larger than the other parts, the hindbrain develops a bulky dorsal extension known as the cerebellum, and the midbrain becomes very small as a result.