As degeneracy checks the natural course of embryonic development it necessarily finds expression in the body as well as in the skull. One most striking condition is that by which development of the bones enclosing the spinal cord is checked. The spinal cord is at first essentially a notochord as in the lowest types of vertebrates. The structures surrounding the cord are not divided into vertebræ. This condition is permanent in the lancelet. Around the notochord is later formed a species of membrane which protects it, called the perichord. This condition is the second stage of development of the cord and is the permanent condition in the lampreys. Later still the cartilaginous vertebræ develop, and then these ossify at the point in the perichord which is to form a vertebra, bows of dense tissue form which unite behind. In front similar bows form to constitute the bodies of the vertebræ. These bows remain ununited in some of the lower fish and at certain stages in the human embryo. As degeneracy checks the union of the bows of the vertebræ, imperfection, and even absence, of the union occur, which is called spina-bifida (Fig. 96). This condition when complete is rarely compatible with life. In a partial state it is often found among degenerates.[220] The seat of the trouble is frequently covered by an excessive development of hair (hypertrichosis), especially in the small of the back; this, which occurs very frequently in degenerates, resembles the tail which the ancients represented as that of the fauns.[221] [Illustration: FIG. 96.] As the vertebræ unite irregularly, deviations or bends of the spine occur very frequently among degenerates. These may be of any of the types known to surgeons. In man the spinal column terminates in two bones. One of those (composed of five vertebræ) begins at eighteen years to unite slowly into a single bone called the sacrum. The bones of the sacral vertebræ form processes similar to those which are formed from the vertebræ of the chest region. These serve to cover the nerves of the sacral region. The bone immediately below the sacrum, called the coccyx, is essentially the representative of the tail in man. At a certain stage of human development, as in the tadpole, the tail disappears, the nine vertebræ forming the coccyx unite together and become a very diminutive bone which loses nearly all vertebral characteristics. Sometimes this bone retains its embryonic peculiarities to such an extent that it simulates in some degenerates a rudimentary tail. Of this many instances are on record. A greater degree of this condition has been found to occur, with comparative frequency, amongst the lowest negro races. In this respect these are below the anthropoid apes, where the tail, considered from the tail standpoint, has degenerated as in man for the benefit of the organism as a whole. [Illustration: FIG. 97.]