1. If the brain is removed immediately after pupation from the silkworm pupae which have been determined to produce the non-diapause eggs the resulting moths lay diapause eggs. 2. If isolated from the brain, the suboesophageal ganglion of the pupae releases a factor responsible for producing the diapause eggs. The “diapause factor” from the suboesophageal ganglion exerts a certain effect on the growing eggs and makes them the diapause eggs. 3. In the determination of the non-diapausing property of the eggs, the brain is of primary importance. It suppresses by way of the oesophageal connectives the suboesophageal ganglion to release the diapause factor.