This is an affection of the chorion, arising by a transformation of the villi. The villi are branching offshoots consisting of mucous tissue, covered by a layer of epithelium. By increase of the mucous tissue the villi are converted into cyst-like bodies of considerable size. The hydatid mole is thus formed of a mass consisting of the foetal membranes, on the surface of which are myriads of oval or round cyst-like bodies which hang like grapes on stalks, and often hang on one another (see Fig. 466). Thus a bulky mass is formed consisting of a multitude of these berry-like cysts. When the altered membranes are opened, a dead and blighted foetus is revealed, generally in an early stage of development.

In an early stage of development the villi are present all over the chorion, while later on they are limited to a certain area forming the foetal part of the placenta. If the disease begin early the cyst-like bodies are present all over the chorion, whereas in later periods they occupy only the placental surface.

The altered chronic villi adhere to the wall of the uterus, and sometimes are even embedded in its substance, so that when removed the internal surface may be like that of the heart, showing prominent trabecular There may even be a destructive encroachment on the uterine wall.

The hydatid mole grows quickly, so that the enlargement of the uterus goes on more rapidly than in a normal pregnancy. The mole is detached at first in pieces, along with haemorrhage, and even when the bulk of it is discharged, it may be so mixed with blood as to be with difficulty distinguished. The clear grape-like bodies afford the means of distinction.

Portion of a hydatid mole showing the berry like masses.

Fig. 466. - Portion of a hydatid mole showing the berry-like masses. Natural size. (Virchow).