148 



SCIENCE 



[N. S. Vol. XLVII. No. 1206 



difference is that tlie eggs remain smooth 

 and spherical during the entire period of 

 shrinkage, the surface showing none of the 

 folds and crenations so characteristic of the 

 fertilized eggs. Evidently the properties of 

 the plasma-membrane have imdergone pro- 

 found changes during fertilization, so that 

 by the above simple osmotic method the 

 fertilized eggs are immediately and sharply 

 differentiated from the unfertilized. One in- 

 cidental effect of the rapid shrinkage of ferti- 

 lized eggs (which involves a corresponding 

 increase in density) is that they sink at first 

 in the hypertonic sea-water more rapidly than 

 unfertilized eggs; and in fact a partial sepa- 

 ration of the two kinds can readily be accom- 

 plished in a test-tube or graduate by taking 

 advantage of this difference in the rate of 

 sinking. 



Shrinkage, if not too pronounced, has little 

 or no injurious effect upon the eggs, and 

 fertilized eggs after return to sea-water con- 

 tinue their development. The osmotic proper- 

 ties of the plasma-membranes are apparently 

 xmaffected by the process of shrinkage. If 

 mixed eggs that have been well shrunken by 

 exposure to the above hypertonic sea-water 

 for five minutes are returned to normal sea- 

 water, it is observed that the fertilized eggs 

 regain their normal water-content much more 

 rapidly than the unfertilized; after a minute 

 in the normal sea-water they are distinctly 

 the larger of the two. Later, as the unferti- 

 lized eggs also approach osmotic equilibrium 

 this difference disappears. 



Experiments similar to the above were also 

 performed with the fertilized and unfertilized 

 eggs of the sand-dollar, Echinarachnius parma. 

 These eggs were found similar to those of 

 Arbacia in their behavior in dilute and con- 

 centrated sea-water. 



Artificial membrane-formation by treatment 

 with butyric acid produces the same kind of 

 change as normal fertilization in the osmotic 

 properties of Arhacia eggs, but the degree of 

 the effect is much more variable. It was in- 

 teresting to fitnd that the rate of shrinkage 

 in hypertonic sea-water shows a definite cor- 

 relation with the character of the membrane 



separated from the e^s'> when this is well 

 separated and sharply defined in appearance 

 the rate of shrinkage, the crenation, and the 

 other features of the behavior approach closely 

 those of sperm-fertilized eggs; on the other 

 hand, eggs with poorly formed membranes 

 show more gradual shrinkage and relatively 

 little crenation; while eggs which show little 

 or no indication of membrane-formation 

 (some of which are always present) show the 

 slow shrinkage and lack of crenation char- 

 acteristic of unfertilized eggs. Such grada- 

 tions of behavior, indicating gradations in 

 the degree to which permeability has been 

 increased by the treatment, are always found. 

 This variability is probably correlated with 

 the variability in the developmental capacity 

 of artificially activated Arbacia eggs, which 

 typically yield only a small proportion of 

 normal larvse. 



The above change in the properties of the 

 plasma-membrane does not take place sud- 

 denly, but begins gradually and requires 

 about 20 minutes after fertilization (at 20°- 

 22°) to reach an approximate final stage. The 

 change is thus progressive and continues long 

 after the separation of the fertilization-mem- 

 brane. At five or six minutes after insemin- 

 ation there is little apparent difference from 

 Tm.fertilized eggs, i. e., the eggs shrink slowly 

 in hypertonic sea-water and do not crenate; 

 at nine or ten minutes shrinkage is con- 

 siderably more rapid and there is some cren- 

 ation; at thirteen to fourteen minutes the 

 majority show well-marked crenation within 

 one minute; at twenty minutes both rate of 

 shrinkage and degree of crenation are still 

 further increased, although the maximimi is 

 not reached until considerably later. It is 

 evident that some special process is initiated 

 by the spermatozoon, having the effect of 

 changing the properties of the plasma-mem- 

 brane in the direction of increased perme- 

 ability to water and increased liability to 

 crenation. 



The intimate nature of this process can not 

 be defined clearly at present, but it was found 

 to be checked or arrested, reversibly, by sin- 

 esthetics or high concentrations of cyanide. 



