Froriep, a follower of Rosenberg, remarks that the lumbo-sacral vertebræ, _i.e._ those constituents of the vertebral column which form the transition from the lumbar to the sacral vertebræ, are invested with fresh interest by Rosenberg’s hypothesis. According to their position in the vertebral column, they are to be regarded as lumbar vertebræ, introduced too early or too late into the structure of the sacrum. If the twenty-fourth vertebra is assimilated with the sacrum, so as to form an upper promontory or outwork, this variety offers a point of transition to a future formation (?) in which this vertebra normally becomes the first sacral vertebra, and the column will now display twenty-three free vertebræ. If, again, this transition occurs in the twenty-fifth vertebra of the series, which thus becomes the chief sacral vertebra, this is, in Rosenberg’s opinion, a characteristic survival of the racial development, an atavism.[50] According to Welcker’s theory, the chief sacral vertebra in one animal corresponds to the same sacral vertebra in another animal, whatever their number may be. The cervical vertebræ of one animal, which may be five, seven, or even eleven in number, correspond to the cervical vertebræ of another animal. The vertebral column of one animal corresponds to the vertebral column of another, taken as a whole, but not to two-thirds or three-fourths of that column. In accordance with the requirements of a given animal, that part of the bone which belongs to the sections of the breast and loins is more or less abundant, and the vertebræ are homologous in accordance with their region, and not with their number.