There are two genera of the omphalositic single monsters, with four species. One of the twins, the autosite, is commonly a normal child; the other, the omphalosite, may be as small as a child's fist, and be very much deformed. Of these omphalosites the _paracephalus_ has an imperfect head, commonly no heart, and the lungs are absent or rudimentary. The _acephalus_ has no head, and commonly no arms; the _asomata_ is a head more or less developed, with a sac below containing rudiments of the trunk organs. The Acephalus is very rare--the rarest of all monsters except the Tricephalus. There is a fourth kind--the _foetus anideus_. This is a shapeless mass of flesh covered with skin. There may be a {76} slight prominence with a tuft of hair on it at one end of the mass to indicate the head. In this monster there are more traces of bodily organs than might be expected. These four kinds of omphalosites are either dead when born, or they die as soon as the placental circulation is cut off. If there is any probability of life, the physician should give them baptism before the placental circulation is stopped. Nothing satisfactory is known concerning the etiology of single monsters. Landau, and other authorities as great as he is, reject the theory that maternal impressions from fright or exposure to the sight of hideous deformity are the cause of terata. I think the father is accountable for terata as often as the mother is. Barnes, an English physician, and others claim they find that terata are frequent in consanguineous marriages, but I have not been able to verify the assertion. It seems a theory may be offered to explain the single terata. In 1888 Roux of Breslau by puncturing one blastomere of a frog's egg in the two-cell stage killed the punctured blastomere without affecting the other. The punctured blastomere remained inactive, but the other developed into a complete _half_ embryo. Crampton by separating and isolating the blastomeres in the two-cell stage obtained a half embryo; and Zoja by isolating blastomeres of the medusae, Clytia and Laodice, got _dwarfed_ larvae. Wilson succeeded by the separation through shaking of the blastomeres in the two-cell and four-cell stages in developing Amphioxus larvae, which were half the natural size for the two-cell blastomeres, and commonly half the normal size from the four-cell blastomeres, yet in the latter some of the larvae were of the normal size but imperfect From the eight-cell stage he got only _imperfect_. larvae. Similar results were obtained by other operators with various eggs. Driesch and Morgan by removing part of the cytoplasm from a fertilized egg of the ctenophore, Beroe, produced imperfect larvae showing certain defects which represent the parts removed.