VARIATIONS IN VERTEBRATE BRAINS

VARIATIONS IN VERTEBRATE BRAINS

. All vertebrate brains share a common undering form, which appears most clearly during early stagee of embryonic development

:.In its earliest form, the brain appears as three swellin

:at the front end of the neural tube; these swel in eventually become the forebrain, midbrain a rhombencephalon) respectively. At the earliest sta equal in size and hindbrain (the prosencephalon, mesencephalon, and of brain development, the three areas are roughly In many classes of vertebrates, such as fish and amphibians, the three parts remain similar in size in the adults, but in mammals the forebrain becomes much larger than the other parts and the midbrain becomes very small. The ratio of brain size to body size also varies considerably across different vertebrate groups Agnathans both lampreys and hagfishes - havé relatively low brain: body ratios, as do amphibians Ray-finned fishes exhibit a wide range

with a few species overlapping the lower

ne range of mammals um. Reptiles fall within a modest range

with both mammals and birds having the

largest brain: body ratios, but a number

of cartilaginous fishes overlap the bird-

enden mammal range. ernen ataThese data also demonstrate the wide

range of variation within multiple groups of vertebrates that is the result of independent evolutionary processes neural tube, called the cerebral vesicle, that qualifies as a brain in terms of its position and several regional features that correspond to those of vertebrate brains.