STRUCTURE AND DEVELOPMENTAL RATE 171 



In comparison with such facts, the occurrence of 9.3 per cent 

 in lot A] and even the 2.3 per cent of doubleness in group Ao would 

 scarcely warrant any other interpretation than that such condi- 

 tions had in some way been induced by the experimental treat- 

 ments. There can be little doubt that the embryonic axis is 

 initially expressed during a very critical and comparatively brief 

 developmental moment. When the axis is once expressed, com- 

 mon observation teaches us that in some way it prevents the oc- 

 currence of other axes or other embryos on the same blastoderm. 

 Doubleness very probably, as will be more fully discussed below, 

 results from the almost simultaneous occurrence of two embry- 

 onic shields instead of one, and this is further due I believe to the 

 probability that neither of the axes possesses the advantages 

 which normally suppresses the expression of other potential 

 budding points. 



To further illustrate the occurrence of doubleness in Fundulus 

 following treatment with low temperature, we may briefly sum- 

 marize one other experiment. 



Experiment 890. These eggs were developed during the early cool 

 part of the season and the control itself progressed rather slowly. 

 The lot Bi was placed in a temperature of 5°C. 3 hours after fertiliza- 

 tion when in an early 2-cell stage. 



Twenty-four hours later the control had developed high segmentation 

 discs which had not yet flattened to cap down upon the yolk-sphere. 

 The night had been unusually cool and these eggs were thus consider- 

 ably retarded in their development. This amount of retardation is not, 

 however, particularly injurious, as is shown by the later development 

 of the eggs. It would seem that Fundulus eggs were sufficiently resist- 

 ant not to be noticeably deformed by the retardations in development 

 induced by the degrees of low temperature which might occur during their 

 spawning season in this climate. Nevertheless, embryos developed dur- 

 ing the early cool part of the season are not so large in size or vigorous 

 in behavior at the time of hatching as are those being developed during 

 the warmer days to follow. 



The eggs of lot Bi after 20 hours at 5°C. are in 2- and 4-cell stages, 

 they are, therefore, almost completely stopped, having divided only once 

 during this time. • 



When 2 days old, the control had the germ-ring only about one- 

 fourth over the yolk sphere, with the embryonic shield beginning to 

 form, a stage not more than one-half as advanced as is usual for this 

 age. Group Bi contained eggs in the first, second, and third cleavage 

 stages with many very irregular arrangements of the cells. These 



