STRUCTURE AND DEVELOPMENTAL RATE 133 



At five days old these eggs were still developing remarkably well 

 although very slowly. The germ-rings were a little further over the 

 yolk. They were now returned to room temperature after having 

 spent 4 days in the refrigerator, 24 hours at 5°C. and 3 days at 9°C. 



One day later, all of the eggs were developing and almost every one 

 presented a well-formed embryo normal in appearance. 



When ten days old, all were living with a fine circulation of the blood 

 and otherwise apparently normal. 



When 17 days old, 18 of these embryos had hatched and 24 were 

 unhatched. 



After 24 days, 12 were still unhatched, one of these being very ab- 

 normal. All of the embryos had seemed normal when ten days old, but 

 at this time it was readily seen that the 12 unhatched specimens were 

 really far behind the control. While showing no gross deformities 

 they were smaller and not so well developed as the control. 



Although these early arrests do not give marked effects on the very 

 young fish, it is certainly possible that many later symptoms might 

 develop if their existence was observed through longer periods of time. 



When 29 days old, 4 embryos were still unhatched, one had died and 

 3 seemed normal and ready to hatch. Thus the record of this group for 

 the length of time it was followed does not compare unfavorably with 

 the ordinary control records of Fundulus embryos up to a comparable 

 period. As might be expected, however, eggs after being 24 hours old 

 which were stopped or retarded in development for 4 days are not 

 able to hatch on schedule time with the control; but are several days 

 late in reaching the hatching stage. 



Such results will be found to differ entirely from those con- 

 sidered beyond as obtained when the eggs are stopped during 

 more critical developmental stages or at times when rapid cell 

 proliferation and developmental changes are occurring. There- 

 fore, it may be stated in general that certain indifferent mo- 

 ments in development do exist during which time the rate of 

 development may be slowed to almost stopping, or development 

 may be actually stopped, and later resumed at a normal rate 

 without causing structural anomalies or unusual conditions in 

 the resulting young fish. 



It is also shown by the above experiment that development 

 may be stopped at certain indifferent periods, in a temperature 

 of 5°C. and then resumed at an extremely slow rate in 9°C. for 

 several days, and later increased to a normally rapid rate at room 

 temperature without injury. 



Thus it is not always necessary that development be promptly 

 resumed at a normal rate in order to avoid structural defects. 



