Last summer I announced1that if an incision is made into the uterus of a guinea-pig at certain early stages of pregnancy, nodules will be formed that have the structure of the decidua of the guinea-pig. Since then these investigations have been continued. In the following I wish to summarize briefly some of the main results obtained: The operative procedure consisted merely in making transverse and longitudinal cuts in the uterus of the anesthetized animal. In order to obtain a large number of deciduomata it is necessary to make many cuts that advantageously separate the continuity of the uterus in different directions. Under these conditions it is possible to obtain a very large number of deciduomata in the uterus of the guinea-pig. The different stages in the development of these deciduomata were established by microscopic examination. In a number of cases serial sections of the nodules were made. These nodules