PHYSIOLOGY OF CELL-DIVISION 381 



after fertilization, but at forty minutes there is already a slight 

 and at forty-two minutes a distinct increase in susceptibility; 

 this rapidly increases to a maximum which coincides with the 

 time at which the furrow is forming in the great majority of 

 eggs. When the furrow is complete the former resistance rapidly 

 returns. Ten minutes after cleavage the condition of the mem- 

 brane is apparently nearly if not quite the same as ten minutes 

 before the first appearance of the furrow. A few susceptible 

 eggs are usually found at this time ; however, and it is possible 

 that after cleavage has once started the interval between cleav- 

 ages is too short to admit of complete recovery of the condition 

 characteristic of the uncleaved egg. 



In the series of table 3 the maximum of susceptibility was 

 found at 52 minutes after fertilization, at a time when the ma- 

 jority of eggs were undergoing change of form. By using less 

 dilute sea-water the precise time of this maximum can be de- 

 termined more accurately, since with sufficiently low dilutions 

 (e.g., 40 to 45 per cent) only those eggs are cytolyzed which 

 have reached the most susceptible stage. Conversely, by using 

 sea-w^ater more dilute than 60 per cent, the preliminary increase 

 of susceptibility can be detected considerably earlier than ten 

 minutes before the first appearance of the furrow. By thus 

 varying the dilution of the medium the progressive character 

 of the change in the plasma-membrane can be demonstrated 

 with great clearness. 



Table 4 gives a summary of all of last summer's experiments 

 which were carried out in a manner similar to the above. The 

 dilutions range from 52.5 to 62.5 volumes per cent. The table 

 gives the times after fertilization at which eggs were placed 

 in the dilute sea-water, and the approximate proportion of eggs 

 found cytolyzed after exposures of respectively three and thirty 

 minutes. In all cases a well defined maximum of susceptibihty 

 was found at the time when the cleavage-furrow was in process 

 of formation in most eggs. The formation and extension of 

 the furrow occupy three or four minutes at this temperature 

 (21 to 23°) (see table 4). 



