Most twins are developed from two ova, and are just as distinct as children who are born one at a birth. But the so-called homogeneous twins are developed from one ovum, are contained in one chorion, and have a common placenta. They are always of the same sex and closely resemble each other, so closely that their individual identity is often mistaken. In a certain sense such twins are examples of double monstrosities.

There are, on the other hand, twins in whom the cerebro-spinal axis is complete in each, there having been a complete division, but union has taken place and the two are connected by living tissue. For the most part the two bodies are placed parallel to each other, and with the anterior surfaces turned towards each other, and they are usually united by their anterior parts. The explanation of this may be that, as the anterior parts are the last to close, adhesion is more likely to take place here. This rule is, however, not without exceptions.

Although the cerebro-spinal axis is complete, there is in double monsters, not infrequently, a suppression of certain parts, the suppressed parts being symmetrical; as if, being in contact, portions had been crowded out by the growing structures.