138 CHARLES R. STOCKARD 



which to stop or interrupt the progress of development. From 

 such experiments one seems justified in classing these moments 

 in development as indifferent at which arrests may be induced 

 without causing subsequent high mortality among the embryos 

 and without a considerable percentage of gross structural de- 

 formities resulting. The eggs treated in the above experiments 

 were all stopped at comparatively indifferent moments in the 

 course of development so far as their gross structure and behavior 

 up to the newly hatched free swimming stage of life would indi- 

 cate. In the section following a review of experiments with 

 decidedly different results will be considered. 



c. Stopping or retarding the progress of development at stages of 

 critical susceptibility to developmental interruption 



From facts we know of development in nature, as well as, from 

 the experiments discussed in the preceding section, it becomes 

 evident that the course of embryonic development need not nec- 

 essarily progress in a continuous manner, but may be stopped 

 entirely for a considerable length of time or may be decidedly 

 reduced in rate without necessarily injuring the end result. On 

 the other hand, it is equally well known in a general way, and 

 even more widely believed, that when a developing egg is injured 

 in such a manner as to cause its development to stop, it is usu- 

 ally incapable of resuming development at all, or if it does start 

 again to develop it will only continue for a short time and often 

 in a very abnormal fashion. 



These two apparently contradictory statements are equally 

 true. This is due to the fact that the way in which a developing 

 egg responds after having had the progress of its development 

 stopped or arrested by any unfavorable condition depends 

 entirely upon the stage in development at which the interruption 

 occurred. In the first case stated above, the interruption is 

 introduced at a stage in development when no unusually rapid 

 changes are taking place, a comparatively quiescent moment 

 during which all parts are developing, but during which no par- 

 ticular or important part is going at an excessively high rate. 

 Such a time we may term a 'moment of indifference.' 



