CELL-DIVISION 



In the chapter, General Properties of the Ectoplasm, 

 was mentioned the method of study of the susceptibility of 

 eggs by putting them in solutions of dilute sea-water. 

 In the experiments of R. S. Lillie^ 40 per cent, sea-water 

 and 60 per cent, tap-water gave a solution that destroyed a 

 percentage of the eggs at once, others later. In my experi- 

 ments- on the rhythmical susceptibility of eggs of sea- 

 urchins to hypotonic sea-water during a cleavage cycle, I 

 modified this method. Instead of exposing the eggs to a 

 solution made up of 40 parts sea-water and 60 parts tap- 

 water, I used much more dilute solutions, mostly one made 

 up of one part sea-water and 90 parts tap-water. On eggs of 

 Echinarachnius I have also used 10 cc. of tap-water to one 

 drop of a sea-water suspension of eggs, an even greater 

 dilution. The value of this method lies in the sharp 

 results that it furnishes. 



I do not wish to imply that I discount the worth of the 40 

 per cent, sea-water dilution as a means of revealing the 

 susceptible periods. On the contrary; for I appreciate the 

 fact that because it destroys eggs exposed to it during such 

 periods, whilst It does not immediately arrest development 

 of eggs exposed during their resistant periods. It Is an excel- 

 lent experimental means. I have used this dilution as well 

 as others successfully; but I have found that its use should 

 be checked by that of the greater dilutions mentioned 

 above, for the reason that eggs exposed to lesser dur- 

 ing terminal stages of resistance prior to the onset of the 

 susceptible period might develop Into this period. In 

 such cases the eggs do not break down in the stage which 

 they had reached at the moment of exposure. The dilu- 

 tions therefore should be so great that they halt develop- 

 ment abruptly; then the time in seconds from the moment 



1 Lillie, R. S., igi6. 

 ' Just, i()22e; ig28d. 



