The synchronisation of hatching in quail embryos: aspects of development affected by a retarding stimulus.

One hundred twenty-eight quail embryos were tested in 33 groups. In each group one egg was stimulated with sounds at a rate known to delay hatching (0.9 clicks/second). In groups of bobwhite quail, stimulation began three days or two days before hatching, and three days before hatching in groups of Japanese quail. All eggs were incubated separately and respiration was monitored to indicate 1) the onset of lung ventilation, 2) the time when the egg began to click, and 3) the hatching time. In one set of bobwhite quail heart rate was also monitored. Stimulated bobwhite quail hatched later than controls, but only when stimulated from three days before hatching. In the Japanese quail fewer embryos were retarded. In retarded bobwhites the duration of breathing, but not the duration of clicking, was lengthened. Respiration rates in stimulated bobwhites were higher throughout stimulation than those of controls. One factor contributing to this increase appeared to be the pacing of breathing in stimulated embryos: breathing tended to stabilise at the level of the stimulation rate. The heart rate of stimulated embryos was higher than that of controls, significantly so around the time of the onset of breathing. In bobwhite, but not in Japanese quail, the increase in respiration rate was found to be associated with retardation and not with stimulation alone; it did not occur in those embryos which were stimulated but not retarded.