By this time the embryo is a minute longitudinal streak at the {74} surface of one pole of the ovum. The "Primitive Trace" is like a long inverted letter U, the legs of which are in apposition. The Primitive Trace becomes a circular flattened disc; and it grows into a cylindrical body by the juncture of the free margins which fold downward and inward and meet in the median line, and this closes in the pelvic, abdominal, thoracic, pharyngeal, and oral cavities. The legs and arms bud from this cylinder later. While the ventral cylinder is growing, another longitudinal cylinder is formed along the upper surface of the embryo, which will contain the brain and the spinal column. The subsequent development of the embryo and foetus need not be known for an understanding of the material considered in treating here of terata. Human terata occur in certain rather definite, types of erroneous development, and the classification of Hirst and Piersol (_Human Monstrosities_, Philadelphia, 1891), which is a combination and change of the classifications of Geoffrey Saint-Hilaire, Klebs, and Förster, is the most satisfactory. There are four great groups of abnormally developed human beings: (1) Hemiteratic; (2) Heterotaxic; (3) Hermaphroditic; (4) Monstrous. Hemiterata are giants, dwarfs, persons showing anomalies in shape, in colour, in closure of embryonic clefts, in absence or excess of digits, or having other defects. This group does not come under discussion here, but attention should be called to the fact that women who are dwarfs are to be warned before marriage that they cannot be delivered normally,--that the caesarean section or symphyseotomy will be necessary, or that certain physicians will practise craniotomy in delivering them. The Heterotaxic group comprises persons whose left or right visceral organs are reversed in position through abnormal embryonic development; the liver is on the left side, the heart points to the right, and so on.