STRUCTURE AND DEVELOPMENTAL RATE 125 



ent in eggs stopped or slowed at different stages that the exact 

 time of treatment will be considered in connection with the dif- 

 ferent effects obtained. The difference in effects between slowing 

 and actually stopping development will also be considered. 



Other eggs were crowded close together in bunches and de- 

 veloped in bowls at room temperature. The eggs near the center 

 of the masses or bunches obtained much less oxygen and were in 

 a higher concentration of CO 2 than the more superficial ones. 

 These were slowed in their rate of development. Sea-water was 

 boiled so as to drive out most of the air and afterward kept stag- 

 nant. Egg masses were developed in this water and the inner 

 eggs of the mass were almost completely stopped in many cases. 

 In all such arrangements the rate of development was so retarded 

 that many abnormal and deformed embryos resulted. 



These in general are the methods employed; the different 

 times of application and the results will be discussed in the par- 

 ticular cases below. 



b. Stopping or retarding the progress of development at stages of 

 apparent indifference to such interruption 



In order to successfully change a continuous into a discontinu- 

 ous mode of development, without producing ill effects on the 

 resulting embryos, it becomes necessary to locate certain indiffer- 

 ent periods during embryonic development at which the inter- 

 ruption may be induced. Certain of these indifferent periods 

 are those moments at which the interruptions of development 

 occur in nature. Should the stoppage naturally take place dur- 

 ing a sensitive period, the species would readily be eliminated 

 on account of the high proportion of abnormal embryos which 

 would result. 



When the eggs of Fundulus are placed in low temperatures 

 after having passed through the earliest active stages of devel- 

 opment, cleavage, gastrulation, the formation of the germring 

 and early appearance of the embryonic shield, they may be 

 stopped for several days, or caused to develop at an extremely 

 slow rate, without marked injury to the resulting embryos. In 

 fact, when such eggs are returned to room temperature after 



