If fission is complete homogeneous twins are the result; these twins are of the same sex and very similar in appearance. Incomplete fission, as has been said, gives rise to double or triple terata. If one of the teratic twin embryos is stronger than the other, the various combinations of enclosure and parasitism may result, although the origin of parasitic double terata is not convincingly clear. A triple {79} monster, according to the fission theory, arises from a double incomplete cleavage of the primitive trace. Dr. Ephraim Cutter has observed teratic composite spermatozoa which, he thinks, probably have influence in producing composite monsters. There are three orders of the double autositic monsters: _Terata Katadidyma,_ in which the embryonal fission was at the cerebral end; the _Terata Anadidyma_, divided below; the _Terata Anakatadidyma_, divided above and below, but joined at the middle of the body. There are four genera of the Terata Katadidyma with many species. The first genus is the _Diprosopus,_ the double-faced. The doubling varies from the finding of two complete faces to a slight trace of duplex formation in one head. Förster in 500 human monsters observed 29 cases of diprosopi. There are six species of diprosopi: 1. _D. Diophthalmus,_ which has only two eyes, but there is a doubling of the nose. 2. _D. Distomus_, which has two mouths, two lower jaws, two tongues, one pharynx, and one oesophagus. 3. _D. Triophthalmus_, which has three eyes, and the doubling of the face is more complete. There are only two ears. 4. _D. Tetrophthalmus_, which has four eyes and two well-separated faces. 5. _D. Triotus_ is like the last, but it has three ears. 6. _D. Tetrotus_ has four ears, four eyes, and there is some doubling at the pharynx. Two oesophaguses enter one stomach in this species commonly. D. Tetrotus is rare--only one example in man is known. In all diprosopi there is only one trunk, one pair of arms, and one pair of legs. Sir James Paget had a photograph, made in 1856, of a living diprosopus, the second face of which had a mouth, nose, eye, part of an ear, and a brain (?) of its own. The two faces acted simultaneously, suckled, sneezed, yawned together. Are diprosopi twins? An answer to this question will be clearer after a description of other composite terata. The second genus of the Terata Katadidyma is the _Dicephalus_. This genus comprises five species, which have in each case two heads, with separate necks commonly. There are two vertebral columns, which usually are separate down to the sacrum, and they converge at the lower end. {80} In the interior organs doubling will be found corresponding to the degree of separation of the trunks. In all the species of this genus there are one umbilicus and one cord.