THE PHYSIOLOGY OF CELL-DIVISION 709 
injury to the egg. In normal fertilization the evidence indicates 
that the condition of increased permeability lasts for a certain 
time—roughly fifteen minutes—after the contact of the sperma- 
tozoon, and is then succeeded by a period of normal permeability 
(like that of the unfertilized egg) which lasts until the appear- 
ance of the cleavage furrow about half an hour later. If the varia- 
tions in the state of permeability could be graphically represented 
by a curve (with degree of permeability as ordinates and time as 
abscissae) the latter would probably rise rapidly immediately after 
the contact of the spermatozoon and fall again toward normal 
soon after the spermatozoon had completed its entrance; it would 
then tise again temporarily at the time of the first cleavage, and 
similarly with each succeeding cleavage. Presumably any par- 
thenogenetic treatment—if the best results are to be attained— 
should approach the normal in the time-relations of the permea- 
bility-changes which it produces. 
The optimum duration of the exposure to hypertonic sea-water 
after membrane-formation by salt solutions is about twenty-five 
or thirty minutes at normal summer temperature (20° to 23°); 
twenty minutes is too brief and thirty-five minutes too long. 
If we assume that the next sudden change in external conditions, 
the transfer from hypertonic to normal sea-water, momentarily 
increases permeability, we should expect that the closest approach 
to the normal conditions would be obtained if the time of this 
transfer were to coincide with the time when the permeability 
normally undergoes a second increase, 1.e., the time of appearance 
of the first cleavage-furrow. The above time-relations indicate 
that this is the case, and the agreement may be more than a mere 
coincidence. ‘These considerations suggest that the processes 
occurring in the egg during the stay in hypertonic sea-water are 
essentially similar, both in rate and character, to those taking 
place in normally fertilized eggs during the half-hour preceding 
the appearance of the cleavage-furrow. That they are largely 
oxidative in their nature is indicated by Loeb’s experiments, which 
show that the hypertonic sea-water must contain oxygen if it is 
to produce its characteristic effect. 
18 Cf. my recent paper on the present subject, American Journal of Physiology, 
1911, vol. 27, p. 295. 
