1. The structure of the head in 42 cyclopic, synophthalmic, triophthalmic, anophthalmic and acephalic embryos of Limnaea stagnalis, obtained by the action of lithium, has been studied. 2. Besides the displacement, reduplication, fusion or reduction of the eyes, the following deviations of the normal structure of the head have been obtained: a shortening of the cerebral commissure, with fusion of the right and left cerebral ganglion in the median plane of the head; suppression of the anterior and posterior part of the apical plate, of the head vesicle and velum; and fusion of the lateral tentacle fields into a single median tentacle field. Only in the acephalic embryos, further reductions of head organs (nervous system, statocysts, pharynx) occur. 3. The suppression of the differentiation of the 4 posterior cells of the apical plate occurs with absolute constancy in these embryos. In normal development, these cells are derived from the apical cells, which surround the animal pole of the egg. 4. The results do not lend support to the hypothesis that interactions comparable to the "embryonic induction" known in other groups play a part in the development of the head of Limnaea. 5. They prove, however, that lithium acts on a gradient-field in Limnaea as it does in the echinoids and the amphibia. 6. Recent experiments on the direct effects of lithium on the eggs of Limnaea are discussed.