In cases of two embryoes originating from one ovum, which, as .already described, gives rise to homogeneous twins, separate or united, one of the embryoes may develop normally whilst the other is faulty. In the case of ordinary homogeneous twins there is only one chorion, and consequently a single placenta, but each foetus has its own district. It may happen that one of the embryoes, by reason of greater vigour in its cardiac contractions, may largely monopolize the placenta, and so cause atrophy or death of the other, or death may occur from other causes. In either case the defective or dead foetus is crushed by the vigorous and growing one, and is flattened out and atrophied. At the period of birth there may be only the parchment-like remains of the defective twin, the so-called Foetus papyraceus, which is generally born subsequent to the perfect one.

Again, in a twin-foetation one of the embryoes may completely monopolize the placenta, this occurring, according to Ahlfeld, by a more vigorous growth of the allantois in the one, so that it takes up the whole chorion to the exclusion of the other. The allantois of the less vigorous embryo may attach itself to the placenta of the other, or even to the umbilical cord, and the foetus, although completely separate from the other, will depend on it for its blood supply, and will only receive blood which has passed through the body of the more vigorous twin. The result is an exceedingly defective formation, which may be in various degrees. As the circulation is carried on by the more perfect twin, the defective one has no proper heart, hence this group is frequently designated Acardiacus. According to Hirst and Piersol, there is sometimes a rudimentary heart. Hence they suggest the name Omphalosite ( = umbilicus), as the monster is attached by the umbilical cord to the other.

A different origin for these forms has been suggested by Cleland. He supposes an abcaudal or abcranial fission, and that the smaller part has been broken off from the larger so as to become independent.

Acephalus

This form presents various degrees of development of the body and limbs. There may be only pelvis and legs, or there may be part of the vertebral column, and even thorax and arms. The legs and arms, if present, are often coalesced. There may even be some trace of cranial bones. The lungs and heart are always absent.

Acormus (Smaller Part Completely Separated)

In this form the separated portion consists of a head either with no spinal column or a short piece. ,The head itself is ill formed, the parts frequently much altered. The umbilical cord proceeds from the neck. There is, of course, no heart, as there is no trunk.

Amorphus

In this form there is a rounded mass covered with skin, and showing externally no indication of parts of the body. It contains internally fat and connective tissue, with a rudimentary vertebral column. Sometimes there is more of an approach to human form, and the mass contains distinguishable rudimentary parts of the body.