MORPHOGENESIS AFTER HEAT SHOCK DURING CELL-CYCLE OF LYMNAEA - NEW INTERPRETATION

The effect of a heat shock (37.0-38.0.degree. C, 10 min) during the 3rd and 4th cleavage cycles of Lymnaea was investigated. Sensitivity with respect to the duration of the cell cycle and morphogenesis appeared to be periodic. The cycle extension curve had 3 maxima: at the beginning of the cycle, at the G2-phase and at prometaphase. With regard to morphogenesis, the eggs became sensitive shortly before cleavage, when cleavage could not be delayed any more. In eggs treated at the morphogenetically sensitive stages, mitotic abnormalities caused by an incomplete separation of the chromosomes during treatment were observed. Some cells were lethally affected, and the division chronology was abnormal in some embryos. Heat shock probably disturbs a process relevant to the cell cycle. If applied before metaphase, an extension of the cell cycle permitted a complete recovery and morphogenesis remained unaffected. If applied at metaphase or later, cell division was not delayed, but mitosis was seriously disturbed. This irreversible damage was the cause of abnormal morphogenesis. The type of malformation depended on the prospective significance of the affected blastomeres.