50 C. M. CHILD. 



hand the fact that the vacuole remains in its normal position at 

 least as often as it enters the swelling indicates that the swelling 

 is not simply the result of the enlargement of the vacuole and 

 consequently stretching of the body wall. It should also be 

 noted that when the swelling appears the ectoplasm does not 

 merely stretch, it loses its structural features, including the 

 cilia and the entoplasm which the swelling contains differs in 

 appearance from that in other parts of the body. And finally, 

 the vacuolar mechanism is very sensitive to cyanide, the pulsation 

 ceases soon after the animal is placed in the solution and the 

 vacuole slowly undergoes enlargement and sooner or later collapses 

 and disappears. The vacuole region is apparently a specialized 

 contractile mechanism and is undoubtedly the seat of a relatively 

 high rate of dynamic activity. In short the only logical conclu- 

 sion is that the greater susceptibility of the region where the 

 swelling appears results from greater dynamic activity. 



But a difference of some sort must exist between anterior and 

 posterior vacuolar regions, for it was pointed out above (p. 47) 

 that in the lower concentrations of the reagent only the anterior 

 vacuolar region shows a swelling and that the frequency of 

 swelling in the posterior as well as in the anterior vacuolar region 

 increases as the concentration increases. Evidently the posterior 

 vacuolar region is for some reason less susceptible than the 

 anterior and a higher concentration is necessary at least in the 

 great majority of individuals, to bring about swelling in it than 

 in the anterior region. 



A comparison of the rate of pulsation in anterior and posterior 

 vacuoles affords a clue to the nature of this difference. I have 

 found that in general the rate of the anterior vacuole is con- 

 siderably higher than that of the posterior. Sometimes the 

 ratio is as high as 5 : 3, in other cases less, but the difference 

 in rate exists, at least in the great majority of individuals- 

 Evidently the posterior vacuole has a lower rate of dynamic 

 activity than the anterior, and the difference in susceptibility 

 of the two vacuolar regions is undoubtedly associated with this 

 difference in rate. In the lower concentrations or in individuals 

 with a low susceptibility the special susceptibility appears as a 

 rule only in the anterior vacuolar region because only here is 



