An account was previously given of the discovery of two cases of placental fusion in mice, and it was suggested that this condition might occur fairly frequently. This has been amply borne out by subsequent experience, as fourteen other examples have been found while working with mice, and in addition it has once been found in the rat. From this additional material further information about the types of developmental abnormalities that may result from this condition has been gathered. Since the last paper, Owen (1945) has found that commingling of the fœtal circulations in cattle twins can result in the transfer of erythrocyte precursors between partners, resulting in animals exhibiting blood-groups bearing no relation to their genotype. Similar transfers in mice would lead to confusion in many genetic experiments, especially in transplantation experiments with tumours.

No genetical basis for the condition is apparent from the data available. It has been, noted in three matings of CBA mice, two of C3H mice, eight C3♂ × CBA♀, and three stock mice, the proportions being roughly related to the relative numbers of each type examined at the time of littering. The frequency is rather less than 1 per cent, of all placentas seen. The only common feature, to which attention was also drawn in the previous paper, has been that the fusion occurred always in conditions of uterine crowding due to large litter size. The smallest litter size in which this fusion was found was five, but in this case five placentas were present in one horn. Whenever labour was observed, it appeared to be difficult and prolonged. Once, by good fortune, delivery was timed approximately. A CBA mouse was found to have delivered two young, and two more were born within the next ten minutes. Nothing further appeared for two hours and ten minutes, when a live pair with fused placentas was delivered with some distress. The last two of the litter, making eight in all, were delivered within twenty minutes of the completion of the birth of the fused pair