INFLUENCE' OF HYDROGEN ION CONCENTRATION. 329 



the resistance of the Astcrias egg is much lower 10 minutes after 

 fertilization than it is later on. There is a gradual increase in re- 

 sistance, particularly in alkali, from 10 minutes after fertilization 

 until the first cleavage, at which time the maximum resistance ap- 

 pears to be reached. Reference to Figure I will show that de- 

 velopment is inhibited at pH 9.2 if the eggs are transferred to this 

 solution 10 minutes after fertilization; but if the eggs are not 

 transferred until the majority have reached the two cell stage 

 (i.e., about 140 minutes) they not only tolerate pH 9.2 but they 

 tolerate equally well pH 10.0. This difference in resistance is 

 strikingly shown by the cultures 24 hours later. When placed, 

 for example, in pH 9.2 10 minutes after fertilization the velocity 

 of development is greatly reduced ; in many eggs the nucleus di- 

 vides without cytoplasmic division ; fragmentation and abortive 

 division are predominant and no egg progresses beyond the 32- 

 cell stage. When placed in this same solution after the first 

 cleavage has occurred 90 per cent, of the eggs will develop to 

 practically normal swimmers. The increase in resistance to acid 

 is considerably less, the limiting acidity shortly after fertilization 

 being pH 5.6, and 140 minutes after fertilization pH 5.4. The 

 increase in resistance to alkali which gradually appears as the egg 

 approaches the first cleavage plane is not to be confused with the 

 period of great susceptibility which follows the event of fertiliza- 

 tion, or with the periodic changes in resistance to various destruc- 

 tive agents which a number of observers have shown to be associ- 

 ated with the process of cleavage. The shortest interval after fer- 

 tilization at which we transferred the eggs to the pH solutions was 

 10 minutes, and therefore the period of great susceptibility to de- 

 structive agents immediately following fertilization was avoided. 

 And since in our experiments the eggs are left in the pH solu- 

 tions until the conclusion of the experiments, which cover in the 

 case of Asterias 5 cleavages, any periodic fluctuations in resistance, 

 if they occur, are translated into a mean. 



There is little increase in the velocity of division in alkaline 

 solution, 2 or 3 per cent, being the maximum observed in any of 

 our experiments. There is frequently a marked stimulation 

 shortly after transferring to the alkaline solution, but this is transi- 

 ent and is followed by a decrease in the velocity of division, so 



