Total RNA from an equal number of early embryos, engaged in the rapid cleavage divisions (lane 1), early cycle 14 embryos in interphase (lane 2), embryos engaged in embryonic cell divisions 15 and 16 (lane 3), and older embryos where cell divisions are restricted to the nervous system (lane 4) were probed on Northern blots with the cyclin A cDNA. The estimated size (kb) of the cyclin mRNAs are indicated on the left side. Embryos were collected and aged at 25°C for the times indicated and staged under the microscope before isolation of RNA. In order to visualize the signals from cyclin A mRNAs in older embryos, a longer exposure is shown in the lower panel.