The amnion may be the agent in producing malformations in two ways, namely, by adhesion to the developing parts, or by pressure on the parts so as to hinder their growth. The amnion is formed by a backward projection of a double layer from the blastoderm close to the embryo. This occurs first at the cephalic end, and then a very little later at the caudal extremity. The projecting folds come together behind so as to enclose the embryo in a sac, in which, however, the anterior (or ventral) aspect is not included. The embryo gradually sinks into this sack as the amnionic fluid accumulates.

The developing amnion is most closely related to the cephalic and caudal ends of the embryo, and any defect or lesion referable to it will affect especially the head and especially the face, or the pelvis and lower limbs. Adhesion of the amnion to the embryo will necessarily interfere with the active cells in their constructive process.

As the adhesion of the amnion is an accidental circumstance, the resulting lesions are likely to have an irregular character. This is shown in Fig. 4. In this case the amnion is seen to be adherent not only to the head, but to the heart, which is drawn out of the body with its apex upwards. There is great defect in the head and trunk, due to imperfect closure, and there is protrusion of brain, heart, liver, stomach, and the greater part of the intestine. Again, an amnionic adhesion may be drawn out into a cord, and this by constricting parts, such as the neck or limbs, may lead to strangulation, imperfect development, or even to amputation of parts. But, further, the mere non-expansion of the amnion may by its close contact, without adhesion, seriously interfere with the development of parts, and may have the effect of destroying certain of the developing structures. This may explain certain defects of the face, such as cyclopia and agnathia. It also explains the various degrees of deformity of the lower extremities included in the group of siren-malformations.

Fig. 4. - Adhesion of Amnion. Protrusion of brain, heart, liver, stomach, and intestine. (W. I. M).

It is to be remembered that these interferences by the amnion are likely to occur at an early period, when it is in process of formation or shortly after. At these early periods a minute group of cells may represent a considerable portion of the future body, and a slight interference may lead to marked deformity. It will be noted also that the defects will be mostly in the middle line.