256 CHARLES R. STOCKARD 



the development of the fish's egg may be stopped for a considerable 

 length of time with impunity, no ill-effects resulting. This is the 

 developmental moment at which the bird's egg is normally stopped 

 on account of the fall in temperature experienced after passing 

 out of the mother's body. 



There are other stages during which a temporary interruption 

 of the developmental processes will be followed by most disas- 

 trous effects. These critical stages are usually moments during 

 which marked inequalities in rate of cellular proliferation are tak- 

 ing place in different portions of the blastoderm or embryo. The 

 period preceding the process of gastrulation is just such a critical 

 moment. 



2. There are considerable differences in effect between greatly 

 reducing the rate of development and actually stopping the proc- 

 ess temporarily. The development of certain eggs may be 

 slowed down to one-tenth or one-twentieth of the usual rate and 

 be maintained in such a slow condition for days without the major- 

 ity of specimens losing their power of regaining the normal rate 

 and giving rise to structurally perfect individuals. If at similar 

 stages the development of the same eggs be completely stopped 

 instead of slowed down, they are in' many cases unable later to 

 resume the process and die, in other cases they may resume devel- 

 opment in a most abnormal fashion, or finally a few may be 

 capable of resuming the apparently normal process. 



This difference in results between a severe reduction in devel- 

 opmental rate and an actual temporary stop is to be explained as 

 follows: Slowing does not completely eliminate the normal in- 

 equalities in rate of developmental change existing among the 

 several parts. Those parts that were in states of rapid develop- 

 ment are depressed in the same proportion as other parts that 

 were developing more slowly and inequalities in rate still exist in 

 the slow-going embryo. When such specimens are allowed to 

 resume a faster development the several portions of the embryo 

 are able again to maintain normal differences in developmental 

 rate and a proper balance is assured. 



A complete stop in development reduces the rate of all parts to 

 zero and eliminates normal inequalities. On resuming develop- 



