ENVIRONMENTAL AND HORMONAL-CONTROL OF THE SEASONAL EGG-LAYING PERIOD IN FIELD SPECIMENS OF LYMNAEA-STAGNALIS

A study was made in field specimens of L. stagnalis of the relationship between environmental factors (temperature, photoperiod, and food) and spontaneous ovipository activity as well as oviposition evoked by injection of the ovulation hormone (CDCH) during a 1 yr cycle. Apparently, spontaneous egg laying started in mid-May and ended in mid-Sept. It is the quantity of assimilated food that triggers the onset of the egg laying season. Its termination is very likely determined by a synergistic action of environmental factors. Evidently, these factors control the activities of the CDCH-producing neuroendocrine caudo-dorsal cells. In winter the snails are completely insensitive to injected CDCH, although many large oocytes are present in the gonad. During 2 mo. before and after the egg laying season, many injected snails respond to CDCH. These phenomena are caused by changes in the activities of the endocrine dorsal bodies, which control vitellogenesis and the activities of the female accessory sex organs. The changes in the activities of the dorsal bodies are probably controlled by the synergistic actions of the previously mentioned environmental factors. Oviposition latency (interval between CSCH injection and start of oviposition) is much shorter during the egg laying season than in the nonreproductive period. This must be ascribed to the direct effects of temperature.