Studies on the control of spermatogenic DNA synthesis by the mesocerebrum in the snail Helix aspersa.
1. The results obtained after the removal of the supraesophageal part of the circumesophageal neural ring indicate that this part of the brain is involved in the control of spermatogenesis. In hibernating snails, when the supraesophageal ganglia are removed or disconnected from the subesophageal ganglia and from the cerebral sense organs, an increase in the rate of 3H-thymidine incorporation in the male sex cells in the gonad occurs. This suggests that while the supraesophageal ganglia impart an inhibitory influence on the DNA synthesis during spermatogenesis, the subesophageal ganglia stimulate it. 2. The microsurgical removal of different parts of the supraesophageal ganglia suggests that the mesocerebrum plays a major inhibitory role on spermatogonial multiplication. This inhibitory activity originates from groups of mesocerebral neurosecretory cells. These cells have cellular connections to the rest of the periesophageal nerve ring and with the endocrine cells of the dorsal bodies (DB). 3. The extirpartion of islets of neurons located near the cerebral commissure or the section of their axons which form synapse-like-structures with the DB induces an increase in the incorporation of 3H-thymidine by the male sex cells in the gonad. In hibernating adult snails these experiments indicate the inhibitory function of groups of neurons from mesocerebrum on DNA spermatogenic synthesis induced by an increase in temperature (5 to 25 degrees C). In the young snails, this area is the source of growth hormone.