To test whether the perivitelline chamber expands continuously or plateaus in size after initial expansion,we compared the fits of linear and logistic models with measurements of the perivitelline chamber over time. A four-parameter logistic model including clutch (AIC = 19.83, R2 = 0.97) fitted the data better than did either a logistic model excluding clutch (AIC = 250.43, R2 = 0.82) or a linear model including clutch (AIC = 356.4, R2 = 0.70), indicating that the perivitelline chamber plateaus after an initial expansion (Fig. 1).