136 CHARLES R. STOCKARD 



Experiment 908. Specimens 72 hours, or three days old, with the 

 optic cups ah'eady invaginated and formed, but just before the begin- 

 ning of a heart beat, were carefully selected, so that every individual 

 was normal and good, and arranged in two groups. Group Fi, consist- 

 ing of 62 vigorous specimens, were placed in the refrigerator at 5°C. 

 and group F2, containing 36 normal embrj'^os, were subjected to a tem- 

 perature of 8°C. 



When 6 days old and after being 3 days in the refrigerator the Fi 

 lot were in much the same condition as when put in the cold, the 

 hearts had not begun to beat and the general structural appearance 

 had not changed. The F3 lot were a little further advanced, but 

 there was still no heart-beat. The control embryos at this time 

 have, of course, a vigorous circulation of the blood, they are well pig- 

 mented and the yolk vessels are mapped out by the chromatophores. 



At 8 days old, the Fi group were still in the same condition as when 

 put in the 5°C. temperature 5 daj^s before. There was no heart beat 

 and the embryos appeared as if about 3 days old. They were now 

 returned to room temperature. 



The F3 lot, after 5 da3^s at 8°C., were further advanced, their hearts 

 were pulsating feebly and very slowh^, blood-cells were formed on the 

 yolk-sacs and masses of blood were frequently observed in the tail 

 regions. These embryos were also now returned to room temperature. 



After being at room temperature for 3 days, with a total age of 

 eleven days, the Fi lot seem recovered and are developing well, though 

 about 4 or 5 days behind the control. All of this lot were living. 

 The F3 lot were also all alive and in apparently perfect condition. 



When 18 days old, almost all of the control embryos had hatched. 

 The Fi lot all seemed normal, but none had hatched, and the same was 

 true of the F3 group. Two days later, however, many had hatched 

 in both lots. Thus they were 3 or 4 days later than the control in 

 hatching, which was a little less than the time they had spent at low 

 temperature. 



Finally, when 27 days old, none of the embrj^os in the two lots had 

 died, which indicates that they were all unusually good specimens. 

 Every one of the 36 in the F3 group hatched, and but 2 in the Fi group 

 failed to hatch, although these appeared normal in structure. 



A complete stop or an arrest in developmental rate of as much 

 as five days after the optic cups are already formed and just before 

 the beginning of a heart beat does not exert an injurious effect 

 upon any organ that would prevent the normal development of 

 the body form or the capacity to hatch and swim freely. 



Experiment 909. Embryos 6 daj'-s old, with fully vigorous blood 

 circulation over the yolk-sac and within the embryonic body, with 

 chromatophores fully migrated and expanded, and with their bodies 

 moving and twitching, were placed in a temperature of 7°C. After 



