INITIATION OF DEVELOPMENT IN EGG OF ARBACIA. 387 



I. 



If the uninseminated eggs of Arbacia be exposed to sea-water 

 made hypertonic by the addition of NaCl or KC1 in the propor- 

 tions 50 parts sea-water plus 8 parts 2 l / 2 M NaCl or KC1, on 

 return to normal sea-water they are induced to cleave and develop 

 plutei. The per cent, of eggs that develop depends upon the 

 length of exposure which will vary somewhat with different lots 

 of eggs. Too brief an exposure will call forth merely the monaster 

 condition and few, if any, of the eggs cleave; too long an ex- 

 posure will produce cytasters, the resulting cleavage being ab- 

 normal. These eggs do not form membranes. 



If 15, 16, and 17 parts zy, U NaCl or KC1 plus 85, 84, and 83 

 parts sea-water, respectively, are employed, the results are similar 

 to those obtained with the hypertonic sea-water mentioned above 

 (in the proportion 8 parts 2^/2 M NaCl or KC1 plus 50 parts sea- 

 water). With hypertonic sea-water made up with 20, 22, and 24 

 parts 2.y 2 M NaCl or KC1 plus 80, 78, and 76 parts sea-water, 

 respectively, however, the results are quite different. In these and 

 stronger hypertonic solutions of sea-water the eggs lift off mem- 

 branes while in the solutions. The time from the instant that one 

 treats eggs with a solution to that at which the eggs form mem- 

 branes will vary with the strength of the solution. Thus in full 

 strength 2^/2 M NaCl or KC1 eggs lift off membranes in 15 sec- 

 onds. In the solution 24 parts 2 l /> M NaCl or KC1 plus 76 parts 

 sea-water the eggs lift off membranes in five to ten minutes. 

 Solutions between these two strengths call forth membranes at 

 rates proportional to the degree of hypertonicity. The rate at 

 which eggs lift membranes while in the solutions depends thus 

 upon the strength of the solution. 



The solution 18 parts 2 l /> J\I NaCl or KC1 plus 82 parts sea- 

 water gives about 3 per cent, membranes. It is thus the minimum 

 concentration for the production of membranes. Hypertonic sea- 

 water below this concentration does not yield membranes. 



On the whole, the optimum concentration is that which gives the 

 highest per cent, of membranes and which likewise allows an 

 exposure longer than that to produce membranes without any 

 deleterious effect on the eggs as revealed by their subsequent fate 



