In these there is a duplication of the whole body, the halves being attached to each other, or a duplication of either caudal or cephalic end. Such monsters are always of the same sex and are usually joined at corresponding parts, as head, thorax, or sacrum. They arise from a single ovum and blastodermic vesicle, and the cause that determines their formation must exert its influence at the earliest stage of development, probably during the formation of the primitive streak and medullary groove.

They may result from:

I. Two embryonic areas arising within a single blastodermic vesicle and continuing to grow.

2. Two primitive streaks and two medullary folds arising within a single embryonic area and either remaining separate or merging.

3. A single primitive streak with either partial or complete doubling of the medullary groove.

4. The duplication may take place late in the development and affect only single parts.

The first three cause abnormalities along the main axis of the body; in the fourth, the variations lie to one side.

Twins and triplets in a way belong to the class of monstrosities, as they are a reversion to the lower types, in which multiple births are common.

The twins may be equally developed and of the same size, or one may be larger and more advanced. They will in the first case usually live; in the second they sometimes continue to exist.

The varieties of double monsters are named by adding the word "pagus" (from pag, meaning "to fasten") to the name of the part of the body by which they are attached, as xipho-pagi, when joined by the xiphoid cartilage; cephalo-pagus if by the heads.

These monstrosities may or may not live, according to the development of the internal organs. In many cases each individual has had well-developed and separate organs and the two have lived for many years.

Sometimes one of the twins may take up the nutrition at the expense of the other, with subsequent increase in size. The larger of the two is called an autosite, and the other a parasite. The latter is generally imperfectly developed.

The abnormalities of the various important organs will be considered in their respective chapters in Part II.