376 RALPH S. LILLIE 



cell-division cycle, but evidently analytical and other data are 

 required to decide this question. The resistance to saponin at 

 different stages has not yet been determined; but Spaulding's 

 observations" show that the resistance to ether declines markedly 

 at the time of cytoplasmic division. 



The resistance of the eggs to osmotic disruption in dilute 

 sea-water thus forms a convenient index of the physical con- 

 dition of the plasma-membrane. As already mentioned, this 

 resistance is lower in the unfertilized than in the fertilized egg 

 (except at the time of cytoplasmic cleavage); i.e., in the unfer- 

 tilized egg cytolysis occurs at a lower degree of osmotic disten- 

 tion than in the fertihzed egg. A further and probably related 

 difference, also mentioned above, is that water enters the un- 

 fertilized egg much more slowly than the fertilized egg; i.e., 

 high resistance to entrance of water — or a relatively waterproof 

 character — appears to involve a relatively slight extensibility 

 of the membrane. 



The degree of dilution required to cytolyze the majority of 

 unfertilized Arbacia eggs in half an hour is about 60 volumes 

 per cent (60 volumes tap water plus 40 sea-water). At 55 per 

 cent dilution most eggs remained intact after 4 hours; at 57.5 

 per cent about half of the eggs were cytolyzed after 1^ hours 

 and all after 4 hours. Fertilized eggs withstand exposure to 

 all of these dilutions for several hours without cytolysis. 



Table 1 gives the results of experiments with different dilu- 

 tions of sea-water. The experiments were performed on dif- 

 ferent days, each with a separate lot of eggs. In each experi- 

 ment fertilized and unfertilized eggs from the same lot were 

 placed in the dilute sea-water — the fertilized eggs about 12 

 minutes after fertilization — and the proportion of cytolyzed 

 eggs was estimated after different intervals. The approxi- 

 mate proportion of eggs found cytolyzed after 20 minutes in 

 the dilute sea-water is given in the table (see table 1). 



The greater resistance of the fertilized eggs to osmotic dis- 

 tension is evident from the table. This high resistance remains 

 until shortly before the appearance of the cleavage-furrow; it 



2' Loc. cit. 



