122 CHARLES R. STOCKARD 



ceed in a continuous manner from stage to stage until the larva 

 or free living embryo is formed, the environment permitting. 

 Stops in development take place through lack of oxygen, 

 unfavorable temperature, insufficient moisture, or shortage of 

 available nutriment, but the egg itself is wound or set for de- 

 velopment so as to continue through if possible. Thus experi- 

 ments on discontinuous development must apply as methods 

 various means for modifying the environment, and the results 

 will depend upon the power of the egg to adjust itself to or with- 

 stand these changes. Being unable to meet the situation, abnor- 

 mal or unusual developmental productions may arise. 



The question then presents itself as to whether the develop- 

 ment of any egg may be interrupted for definite lengths of time 

 and later be allowed to finish or proceed. What would be the 

 consequences of such interruption in the case of a normally con- 

 tinuous mode of development? Would the effects of the manner 

 of development be the same following interruptions at different 

 stages, or would the effects vary depending upon the stage of 

 development at which the interruption occurred? In other 

 words, are there indifferent and critical moments of develop- 

 mental interruption? Would a complete stop in development 

 have an effect similar to a decided slowing of the rate, or 

 would the one be more effective than the other? The experi- 

 ments recorded in the following sections were devised in order to 

 answer these and other queries. 



4. EXPERIMENTALLY CHANGING A CONTINUOUS INTO A DISCON- 

 TINUOUS MODE OF DEVELOPMENT 



a. The method of experiment 



The continuous mode of embryonic development is the more 

 common type in nature. We are, therefore, warranted to some 

 extent in assuming that the discontinuous mode is nature's ex- 

 perimental modification of the continuous. What methods of 

 modification has nature employed that may be artificially imi- 

 tated? The simplest, commonest, and most evident natural 

 method is change in temperature which causes the interruption 

 of development in the eggs of all birds. 



