PHYSIOLOGY OF CELL-DIVISION 379 



resistance or had not wholly lost it; the great majority, however, 

 underwent prompt and complete cytolysis. In the series with 

 65 per cent dilution the results were essentially the same, the 

 only difference being that cytolysis was more rapid, and a larger 

 proportion of eggs exposed in the intervals between cleavages 

 underwent destruction. This is to be expected, since both 

 decline and recovery of resistance are continuous, the former 

 (e.g.) beginning before there is any external evidence of cleavage. 

 Hence at a stage when the eggs are still resistant to the 60 per 

 cent dilution they are destroyed by the 65 per cent. A repetition 

 of the above two series gave the same result. In four other series 

 similar observations were made for the first two cleavages, but 

 not for the third. 



The . change in the susceptibility of the membrane begins 

 several minutes before any external indication of cleavage can 

 be seen. This shows that the change is progressive, reaching a 

 chmax at the time when the cleavage-furrow begins to form. 

 In order to trace its course in more detail, experiments were 

 performed in which eggs were transferred from normal to dilute 

 sea-water at regular intervals of two minutes during a total 

 period of about twenty-four minutes, beginning ten minutes 

 before and ending ten minutes after the period of visible change 

 of form. Eggs were also exposed at two-minute intervals dur- 

 ing the first eight minutes after fertihzation. Table 3 gives 

 the description of a typical experiment of this kind. The pro- 

 cedure was the same as before; sea-water of 60 per cent dilution 

 was used. The first column gives the interval after fertiliza- 

 tion at which the eggs were placed in the dilute sea-water, also 

 the condition of the eggs at that time; the second column gives 

 the approximate proportion of eggs found cytolyzed after ex- 

 posures lasting respectively three and thirty minutes (see table 3). 



These observations show that the eggs resist the disintegrative 

 action of the osmotic swelling until shortly before the first 

 appearance of the cleavage-furrow; they also show that the 

 properties of the membrane begin to change several minutes 

 before there is any visible change in the form of the egg. In 

 the above series the first furrow is seen at forty-eight minutes 



