156 CHARLES R. STOCKARD 



formed embryos was decidedly less from the 9°C. slow lot than 

 in the groups from 5° to 7°C. which had been almost stopped 

 in their development. The 9°C. group also hatched earlier and 

 somewhat better than the other inhibited lots. 



It is thus seen that during the early rather critical stages of 

 development an ahiiost complete stop is much more severe in 

 effect than a decided slowing, on both the resumption of develop- 

 ment and its later progress. An egg developing very slowly but 

 still continuing the process during the early cleavage stages ap- 

 parently possesses sufficient powers of adjustment or regulation 

 to take up a much more rapid development either gradually or 

 rather abruptly. When, as a result of low temperature, devel- 

 opment actually stops during the cleavage or pregastrular stages 

 on raising the temperature, it is frequently stimulated to start 

 again, but the start is so irregular and so out of normal rhythm 

 that many specimens are unable to continue development. 

 These undergo a cellular disorganization followed by death. A 

 considerable percentage of the specimens that do succeed in re- 

 establishing development, still fail to obtain a proper adjustment 

 and balance of developmental activities among their parts. 

 Thus numerous arrested and defective organs are found. This 

 lack of developmental balance among the various parts and the 

 resulting defects are again not so common with eggs that have 

 maintained a continuous development, although for a time it 

 may have been slowed down to an extreme degree. In nature 

 development rarely or never stops during the active early cleav- 

 age stages, though slight temperature changes may frequently 

 cause considerable slowing. The natural interruptions usually 

 occur later, as among the birds, just after gastrulation has been 

 well established. The experiments in previous sections also con- 

 tain data bearing on the effects of stopping and slowing during 

 these later developmental moments. 



Experiment 905 shows the record of two series of eggs both 

 stopped and slowed when twenty-three and twenty-seven hours 

 old, respectively. In both cases the germ-rings were about 

 formed and gastrulation was well on its way. The lots Ci, 

 C2, and C3 after being twenty-three hours old, or in gastrula- 



