STUDIES IN ARTIFICIAL PARTHENOGENESIS. 243 



just described make their appearance 12 or 15 minutes after 

 the eggs are placed in the mercury solution. The constriction 

 of the cytoplasm is not followed by anything approaching normal 

 development, even if the egg is removed from the solution of 

 mercuric chloride and placed in normal sea-water. 



Ordinarily when Arbacia eggs are exposed to an m/ 10,000 

 solution of mercuric chloride in sea-water, only about 10 per 

 cent, of the eggs undergo membrane elevation. Why 10 per 

 cent, of the eggs should throw off perfect membranes and the 

 other 90 per cent, should show no change at all, was for a time 

 a mystery. 



After a number of experiments under diverse conditions, it 

 was found that the percentage of eggs undergoing membrane 

 elevation could be greatly increased by allowing the eggs to age. 

 If eggs are taken from the ovary and placed directly into dilute 

 solutions of mercuric chloride, no membrane elevation occurs. 

 On the other hand if they are allowed to lie in sea-water in 

 shallow dishes for 5 or 6 hours, practically a hundred per cent, 

 will undergo membrane elevation when exposed to the mer- 

 curic chloride. In working with this reagent it is therefore 

 advisable to allow eggs to age before starting to experiment with 

 them. 



The effect of aging is shown in the following table. The first 

 column of this table gives the number of minutes the eggs were 

 allowed to remain in sea-water before they were treated with 

 m/io.ooo mercuric chloride. Only the eggs of a single female 

 were used but these were very plentiful. On removal from the 

 ovary they were placed in 50 cc. of sea-water in a fingerbowl. 

 After a time, as a result of the constant removal of material, the 

 eggs at the bottom of the fingerbowl were no longer evenly 

 distributed, some being more closely clustered than others. In 

 the denser masses of eggs, the aging process apparently proceeded 

 at a slower rate and this accounts for the irregularities in the 

 table. The second column shows the percentage of eggs under- 

 going membrane elevation when subjected to mercuric chloride. 

 To obtain this percentage two hundred eggs were counted in 

 every instance except in the case of the eggs aged for 268 minutes. 

 Only a hundred of these were counted. 



