Morphogenesis after heat shock during the cell cycle ofLymnaea: A new interpretation

The effect of a heat shock (37.0-38.0°C, 10 min) during the third and fourth cleavage cycles ofLymnaea was investigated. The sensitivity with respect to the duration of the cell cycle and morphogenesis appeared to be periodic. The cycle extension curve has three maxima: at the beginning of the cycle, at the G₂-phase, and at prometaphase. With regard to morphogenesis, the eggs become sensitive shortly before cleavage, when cleavage cannot 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.It is concluded that heat shock disturbs a process relevant to the cell cycle. If applied before metaphase, an extension of the cell cycle permits a complete recovery and morphogenesis remains unaffected. If applied at metaphase or later, cell division is not delayed, but mitosis is seriously disturbed. This irreversible damage is the cause of abnormal morphogenesis. The type of malformation depends on the prospective significance of the affected blastomeres.