PHYSIOLOGY OF CELL-DIVISION 383 



In the experiments cited in table 4 the period of maximum 

 susceptibihty varies between 48 and 54 minutes after fertihza- 

 tion; in each series there is a period corresponding to the change 

 of form and lasting about four minutes, during which the sus- 

 ceptibility remains about the same; the resistance then returns 

 rapidly; its return e\ddently signifies the completion of the 

 cleavage-process. With the most dilute sea-water used (62.5 

 per cent) a well marked increase of susceptibihty is apparent 

 twelve minutes before the period of maximum susceptibility. 

 In most series a few eggs remain susceptible ten minutes after 

 cleavage is complete in the great majority; these probably repre- 

 resent chiefly the minority of slowly cleaving eggs; possibly, 

 however, as already suggested, the recovered resistance is not 

 quite equal to that of the uncleaved egg. 



Another and independent method of demonstrating these 

 variations of resistance is by returning eggs to normal sea-water 

 after exposure to the dilute medium for a definite period, and 

 later determining the proportion which continue development 

 to the blastula stage. In several series of such experiments 

 the surviving eggs were always found much more numerous in 

 those lots which had been exposed before cleavage or in the 

 intervals between cleavages; while few if any eggs exposed dur- 

 ing the formation of the furrow continued development. The 

 five per cent or fewer eggs which formed blastulse in Experiments 

 6 to 9 B (table 5) undoubtedly represent the relatively resist- 

 ant minority which had not yet begun to cleave, or had com- 

 pleted cleavage, at the time of placing in the dilute sea-water. 



The degree to which the resistance changes at the time of 

 cytoplasmic cleavage is perhaps best shown by comparing the 

 effects of a graded series of dilutions upon the same lot of eggs, 

 part of these being placed in the dilute sea-water well before the 

 beginning of cleavage (about midway between fertilization and 

 the first cleavage) , and part at the time when the furrow is form- 

 ing in most eggs. Such an experiment shows that at the height 

 of susceptibility many eggs are destroyed by dilutions so low 

 as 40 volumes per cent ; while in order to cause an equally rapid 

 cytolysis in the uncleaved eggs, dilutions of 65 to 70 per cent are 



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