STRUCTURE AND DEVELOPMENTAL RATE 139 



In the second case, the interruption occurs at a time when 

 certain important developmental steps are in rapid progress or 

 are just ready to enter upon rapid changes, a moment when a 

 particular part is developing at a rate much in excess of the rate 

 of the other parts in general. Gastrulation is an important 

 developmental step which apparently cannot be readily inter- 

 rupted without serious effects on subsequent development. 

 Many of the chief embryonic organs seem also to arise with ini- 

 tial moments of extremely high activity, processes of budding or 

 rapid proliferation and growing out. During these moments a 

 given organ may be thought of as developing at a rate entirely 

 in excess of the general developmental rate of the embryo. Such 

 moments of supremacy for the various organs occur at different 

 times during development. As is well known, a certain organ 

 arises much earlier or later in the embryo than certain others. 

 When these primary developmental changes are on the verge of 

 taking place or when an important organ is entering its initial 

 stage of rapid proliferation or budding, a serious interruption of 

 the developmental progress often causes decided injuries to this 

 particular organ, while only slight or no ill effects may be suffered 

 by the embryo in general. Such particularly sensitive periods 

 during development I have termed the 'critical moments.' 



That we may analyze the responses of embryos in which devel- 

 opmental interruptions have been introduced during some of 

 these critical moments, resource may again be had to the records 

 of the experiments. Here also a large number of experiments 

 have been performed, but we shall only attempt a review of cer- 

 tain typical examples from the entitle series. 



Experiment 901, B Series. Eggs were fertilized at 11 a.m., and 

 three hours later, immediately before the first cleavage, they were 

 divided into four lots, one for control and three others which were 

 placed in a refrigerator at temperatures of 5°, 7°, and 9°C. 



When 24 hours old, the control had reached a high segmentation 

 stage, the germ-discs in only a few had flattened down on the yolk 

 sphere, but in none had the cap begun to descend over the yolk or to 

 form the germ-ring. The night had been unusually cool and the 

 control was thus developing far more slowly than the normal sum- 

 mer average rate. At 24 hours old, the germ-ring is usually well 

 formed and has descended about one-third to one-half way over the 



