THE BIOLOGY OF THE CELL SURFACE 



of exposure to disintegration can be attributed wliolly to 

 the dilution's destructive action and not in part to a pos- 

 sible anesthetic action. 



The use of extremely dilute solutions was not my only 

 modification of method. I exposed the eggs during a 

 cleavage-cycle at the short intervals of one and two minutes. 

 Thus, I was able, using always eggs developing at the same 

 tempo, to define with sharpest exactness the briefly endur- 

 ing periods of susceptibility which otherwise would have 

 escaped observation. 



I think that there can be little doubt that my experi- 

 ments did more than confirm earlier ones which proved the 

 existence of periods of resistance and susceptibility during 

 the cleavage-cycle of the sea-urchin &gg\ they demon- 

 strate exactly the onset and duration of the period of 

 susceptibility. The significance of my findings for a 

 theory of cell-division needs no elaboration. If we wish 

 to base a theory of the cause of cleavage upon the occur- 

 rence of rhythmical resistance and susceptibility parallel 

 to the rhythm of cell-division, we first of all need accu- 

 rately to relate the susceptible period to a definite stage in 

 the process of cell-division. One finely spun physico- 

 chemical theory of cell-division falls to the ground because 

 its author failed to time precisely and to relate definitely 

 the onset and duration of that period of susceptibility 

 which precedes the appearance of the cleavage-furrow.^ 



Perhaps the simplest way to begin the discussion of my 

 findings is to present the data of a typical experiment, on 

 the egg of Arhacia punctulata.- Accordingly, these data 

 in the form of a table (Table III) are herewith given. The 

 reader will note: First, there are two periods of susceptibility 



1 Lillie, R. S., igi6. 



^ The egg of Echhiarachiiins which is less resistant than that of 

 Arbacia reveals ninch more sharply the periods of susceptibility. 



274. 



