STRUCTURE AND DEVELOPMENTAL RATE 137 



24 hours the hearts were still beating, but much slower than the con- 

 trol, and they had fallen about 20 hours behind the control in devel- 

 opment. 



After 3 days in the cold these embryos had fallen far behind the 

 control in size and development. The heart was beating slowly and 

 the blood was circulating in all. 



Two daj^s later, when the embryos were 11 days old, they were 

 still in about the 6-day condition, although all were living at a slow 

 rate during the 5 days in the refrigerator. 



When 13 days old, and after being 7 days in the low temperature, 

 the embryos were all alive. They had a slow heart beat and a circu- 

 lation which in many was so sluggish as to allow large sinuses in the 

 3^olk-sac to remain distended with blood, although the circulation 

 within the embryonic body was complete. At this time they were 

 returned to room temperature, and after 24 hours the heart beat had 

 regained a normal rate and the blood was circulating freely and fast in 

 each of the specimens. All seemed fully recovered from the depres- 

 sion caused by the low temperature. 



At 19 days old, almost all of the control embryos had hatched, but 

 none of these that had spent 7 days at 7°C. were yet up to the point 

 of hatching. 



At 22 days old, stijl none were hatched. But when 23 days old, 16 

 had hatched and 38 were unhatched. They were thus 5 days behind 

 the control in beginning to hatch as a result of their 7 days of slow 

 development at the low temperature. 



On the 25th day only 2 were still unhatched, and finally, on the 

 27th day, these two had not hatched, although they seem normal in 

 structure. 



There is, therefore, no evidence that any harm was done by 

 subjecting advanced embryos with blood freely circulating to low 

 temperatures. Although under the cold conditions the heart 

 rate was greatly reduced and the circulation rendered extremely 

 sluggish for a period of seven days. On return to normal tem- 

 perature recovery was rather prompt and seemed on superficial 

 examination to be complete. 



A number of similar experiments to those reviewed above are 

 recorded in my notes, and in all cases the results are in close 

 accord. If we consider them entirely from a standpoint of the 

 external evidence of injury produced, a fair comparison may be 

 made with the results of further experiments in which the eggs 

 were stopped and arrested at other developmental periods or 

 moments. It will be readily shown that periods very close to 

 some of those used above are decidedly dangerous moments at 



