328 HOMER W. SMITH AND G. II. A. CLOWES. 



ments with each species performed during the summers of 1922 

 and 1923. In these experiments the eggs were inseminated in sea 

 water, centrifuged at various intervals after insemination and 

 transferred to the pH solutions. The data given beside the curves 

 show the time after insemination at which the eggs were trans- 

 ferred from sea water to the pH solutions. The development 

 was followed quantitatively on samples taken every 20 minutes in 

 which the number of divisions per egg was determined by careful 

 counts. The mean development was then obtained by averaging 

 all the observations for each solution, and the results expressed in 

 terms of the corresponding figure for sea water as 100 per cent. 

 The full ordinate indicates the H-ion concentration of sea water. 



The dotted line in Fig. I refers to Arbacia and the solid lines to 

 Astcrias. The significance of the wavy portions of these lines 

 will be discussed later. In Arbacia the velocity of division re- 

 mains practically constant from pH 8.15 (the H-ion concentration 

 of sea water) to pH 6.0; at 5.0 velocity of division is reduced 

 by one half and at 4.6 division is completely suppressed. A slight 

 increase in the alkalinity of sea water increases the velocity of 

 division ; this stimulation reaches its maximum about pH 8.8, and 

 amounts to a 15 to 25 per cent, increase over the velocity in sea 

 water. Above pH 8.8 there is an abrupt retardation so that the 

 developmental velocity is reduced by one half at 9.6, and at 10.12 

 only a small fraction of the eggs divide even once. 



Attention is called to the fact that the limiting reactions are 

 characterized, not by a gradual, but by an abrupt inhibition of cell 

 division within a comparatively narrow range. Between these 

 limiting reactions cell division is essentially unimpaired. 



It will be convenient for purposes of reference to define the 

 critical limit as the pH at which the curve under consideration is 

 reduced to its midpoint, i.e., to 50 per cent. Accordingly the 

 limits for Arbacia may be said to be pH 5.0 and 9.6. These 

 limits are the same whether the eggs are placed in the pH solu- 

 tions 10 minutes or 60 minutes after insemination. In the latter 

 case, however, the degree of stimulation by alkali is slightly less. 



The Asterias egg differs from the Arbacia egg notably in this 

 that while the resistance of the latter to both acid and alkali ap- 

 pears to be the same 10 minutes and 60 minutes after fertilization, 



