THE PHYSIOLOGY OF FERTILIZATION 151 



The condition of greater permeability to water does 

 not arise all at once, but gradually during the first 

 fifteen minutes after insemination; this is a matter of 

 great significance in considering the relation of change 

 of permeability to the fertilization reactions. 



The behavior of the fertilized and unfertilized eggs 

 of Echinarachnius to hypotonic and hypertonic sea- 

 water is similar to that of Arbacia; but the starfish 

 (Asterias) shows an interesting contrast to both, as there 

 is little difference between fertilized and unfertilized 

 eggs with reference to exosmosis and endosmosis (R. S. 

 Lillie, 1918). A comparable contrast with reference to 

 oxygen consumption in the sea urchin and starfish eggs 

 respectively has already been noted (p. 145). 



R. S. Lillie (1917) distinguishes two possibilities 

 with reference to the relation which changes of per- 

 meability may bear to the activation process. 



The first possibility is that, after fertilization and as one of 

 the secondary consequences of this process, the general condi- 

 tions of permeability in the egg are permanently modified, and 

 that the protoplasmic surface-layer or plasma-membrane from 

 that time on remains more permeable and more subject to changes 

 of permeability than before; there is, in fact, considerable evi- 

 dence that this is the case. The second possibility, which is 

 the one suggested by the resemblances between the conditions 

 of activation of the resting egg and those of stimulation in 

 general, is that the primary event in the activation process, as 

 well as in the stimulation process, is a temporary increase of 

 permeability; upon this initial change follow the other changes 

 expressive of the general response or activation of the egg-cell. 

 A temporary or initiatory increase of permeability is thus to be 

 distinguished from a permanent alteration in the general proper- 

 ties of the plasma-membrane involving increased permeability. 

 There is good reason to believe that both of these processes are 

 concerned in the activation of the resting egg. 



