26 RALPH S. LILLIE 



If this is true, an externally induced depolarization ought to 

 initiate cell-division in unfertilized eggs; the facts of electrical 

 parthenogenesis thus favor this view. It is well known that 

 cytolytic substances cause in muscle an electrical variation or 

 'injury-current,' similar in direction to the action-current, and in- 

 dicating — according to the membrane theory — increased permea- 

 bility of the membranes ; the same change presumably occurs in 

 egg-cells treated with cytolytic substances. There seems thus to 

 be good reason for assuming that the initial event in fertilization, 

 as in stimulation, is a depolarization-process, which is conditioned 

 by a temporary increase in the permeability of the plasma mem- 

 brane. Since this initial depolarization is in both cases rever- 

 sible, we may conclude that the primary phases of both processes 

 — those in which the membranes are concerned — are essentially 

 similar and differ chiefly in their time-relations, the latent period 

 being relatively long and the rate of the electrical variation rela- 

 tively slow in egg-cells. 1^ There is also reason to believe not 

 only that a depolarization-change is associated with the fertiliza- 

 tion-process, but that a rhythm of alternating polarization and 

 depolarization is similarly associated with the rhythm of cleav- 

 age. ^^ The appearance and reappearance of the characteristic 

 system of cytoplasmic relations, coincidently with the rhythm 

 of cleavage, seems satisfactorily accounted for on the assumption 

 of an alternating polarization and depolarization of the plasma 

 membrane; and the existence in sea-urchin eggs of a parallel 

 rhythm of carbon dioxide production and of susceptibihty to 

 poisons^^ constitutes a further and independent indication of a 

 rhythmical variation of permeability. 



In stimulation the initial depolarization, corresponding to the 

 rising phase of the action-current curve, is promptly and com- 

 pletely reversible, and the same — with characteristic differences 

 in time-relations — is probably true of the fertilized egg-cell. 



" The rate of electrical variation varies widely in different tissues, being most 

 rapid for nerve and least rapid for slowly responding tissues like smooth muscle 

 and heart muscle. 



12 R. S. Lillie, Biological Bulletin, 1909, vol. 17, p. 207. American Journal oi 

 Physiology, 1910, vol. 26, p. 126. 



13 Lyon, American Journal of Physiology, 1902, vol. 7, p. 56 and 1904, vol. 11, 

 p. 52. 



