STRUCTURE AND DEVELOPMENTAL RATE 157 



tion, were almost completely stopped for two days. Their con- 

 dition when returned to room temperature was about the same 

 as when placed in the refrigerator. Development was very 

 promptly resumed at room temperature and only a slight mor- 

 tality resulted from the stopping. Only a few of the embryos 

 showed slight defects, but they were behind the control in time 

 of hatching, on account of the two days' arrest. 



Lots C4 and C5 at twenty-three hours old were placed at tem- 

 peratures of 9° and 10°C. in which they continued their develop- 

 ment at very slow rates, so that after four days the germ-rings 

 had descended over about one-half of the yolk-sphere. During 

 these four days they had advanced in development to a stage 

 usually attained in about twelve hours or were going approx- 

 imately at a developmental speed of one-eighth of the control 

 rate. On return to room temperature these lots quickly resumed 

 the normal rate, suffered no mortality on account of the retarda- 

 tion, and developed into normal specimens which hatched some- 

 what later than the control. These slowed embryos possibly had 

 some real advantage over the completely stopped groups Ci, C2, 

 and C3, though it was only slight if any. 



The D series stopped and retarded at twenty-seven hours 

 old, when in gastrular stages, gave exactly similar records. 

 There was no noticeable excess mortality and no later injurious 

 effects. It may be generally stated that stopping or slowing the 

 development of Fundulus eggs after the gastrular stage, with the 

 temperatures here employed, have no appreciable effect upon the 

 quality of the young fish up to the time of hatching. The mo- 

 ment after gastrulation is established seems generally to be a par- 

 ticularly passive stage at which neither stopping nor slowing 

 the rate of development is followed by injurious results. 



The question now arises whether after stopping development 

 during the gastrular stage there is any difference in result if it 

 recommences rapidly or slowly. When twenty-four hours old, 

 eggs were stopped for one day by cooling to 5°C. They were 

 then allowed to resume development very slowly by being brought 

 into a temperature of 9°C. They developed at a very slow rate 

 for three days and were then brought into the room temperature 



